Or probably it is multi-sig, or used other methods such as Shamir's Secret. I don't think that the amount of bitcoin they hold would have been access by just one person only. Or maybe they can this cold storage: Peace-of-mind cold storage of crypto assets in the Bitcoin Suisse Vault.

Using a hardware wallet will be the best for convenience and safety in order to save this amount of bitcoin. Having two wallets or more for it depends on individual opinions, but having hardware wallet is the main thing and having a better backup of your seed phrase and passphrase in a way not vulnerable to attackers and damages.

You can triplicate the backup and store in in three different locations be it on a paper or metalic plate like you suggested, but using the best option you know will be good. Like me, melatic sheet are the best in a way it can be stored also in places very safe.

The most stupid thing to do is to store your backup in a safe. What if thieves even visit someone, the most vulnerable place to store your backup is the safe you suggested as it is the first place in mind of offline attacker like thieves. You have to keep it in a place people can not know about it, like buried underground or stored inside your home wall.

Numerous custodial services that has been hacked or freezing bitcoin users accounts has been enough reasons we should not use custodial services, we should be careful of custodial services.

Hardware wallet is enough to save 100 btc if care is taking of,  but I will prefer to have more hardware wallets for that. Also be very careful of attackers and damage and never let my private key to touch online.

You can read this about Microstrategy
https://www.google.com/amp/s/www.coindesk.com/bitcoin-business-accounts-unchained-capital%3famp=1

I have a mix strategy of both option you indicate. I put 50% of my portfolio on a custody exchange with an insurance feature such is binance and the rest goes to my hardware wallet. IMHO, Its a waste of profit opportunity to store all my BTC on a hardware wallet and let it sleep there. I'd rather store my money in the bank if I will just make it dormant for a long period of time.

The main reason was big money in BTC is very prone to loss due to volatility of the market. So If you store 5M USD in the hardware wallet then the price drop down 10% in a day or worst, Do you think you can handle the loss? It will be different base on the amount accordingly.

I meant that I trust an open-source project more than a device, a company sold me, that I cannot examine how it works inside. Even if everyone says it. Even if no one had problems with hardware wallets before. I don't understand why I should have this "trust" on my mind. On softwares, code talks.

Not always. If I win the lottery tomorrow, I am in danger whether I want it or not. The winner will be announced and some strangers will know that I'm a millionaire. Similarly, if I have 271BTC ($5m) on my electrum I'll have to be really careful. It won't be announced, but if I cash out $100k, the news will start to spread.

But you need to examine the hardware for which you're running it in. Most of the hardware wallet's firmware are open sourced and you can also sideload them. In the hardware aspect, I'm pretty sure the machine you're running Electrum on is not open sourced as well. If having open source and viewable components is favorable to you, have a look at ColdCard. Having a cold storage wallet does not mean that it'll be completely safe from attackers, physical attacks can still take place with relative ease.

I think it's debatable (being over-paranoid or not) whether sidechannel attacks are possible with the hardware that you're running in. But there are some vectors which hardware wallets are designed to be hardened against while software wallets cannot guard.

Electrum is great for a normal use, other than the pretty serious vulnerability which affected JSONRPC using CORS preflight request of your browser. I don't think it really affects the discussion on cold storage wallet but it really reinforces the fact that having open source software and reviewing it doesn't mean that you can spot all the vulnerabilities. You must be able to trust yourself to review each line of the code diligently and understand what they're doing to ensure that it's free of zeroday exploits.
 

I will suggest to check and read this protocol: glacierprotocol.org
since you will find a step by step guide to setup the strongest cold wallet that you can realize.
you can also find some useful hints or tips that could help to prevent mistakes while create/store your private key.

They're available on github:
https://github.com/trezor/trezor-firmware
https://github.com/Coldcard/firmware

I think someone has actually built a Trezor from the parts they acquired. You'll require some skills though. Electrum's code may be open sourced but it won't mean that the hardware you're running on is free from backdoors (I don't think you'll want to audit the OS, it's very complex). Side channel attack may not seem as much of a concern for most but it's reasonable to assume that it's still a viable attack vector (for which most hardware offers little to no protection from them).

As with the device used to connect the hardware wallet, if you can verify that the firmware does not transmit any private key nor modify any raw TXes to a malicious address intentionally, that should be fairly safe.

I trust Electrum as much as you but there are also hardware wallets which are open sourced as well. If both the firmware and the application is open source, then you'll probably want the machine that you're working on and it's firmware to be open sourced as well.

Hardware wallets, with the inclusion of a secure chip, makes it harder for the attacker to compromise the device given it's various protection against the attack vectors (mostly sidechannel).

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I don't think you can compare the security that a hardware wallet can provide against the myriad of attack vectors against a cold storage wallet. I don't dispute that it's difficult for someone to compromise your Electrum (through compromising your offline OS) but given physical access, it's much harder for someone to breach a hardware wallet than an Electrum wallet. That, and it's smaller margin of error is where HW wallets excel at.

Interesting. I guess option one is far far safer than any other way out that might be present out there. Nano ledger so far is the only physical entity which crypto space has where you can store the coins and be assured you and only you hold them and has access to. Though they are virtually over blockchain but without the hardware connection there is no way to access them. Exchangers, considering their past are the devil's. They have been hacked so many times and billions of have been stolen from their depository. So it's risky best you can ever play. With the 100 million, dam that's just terrific. Lolz.

In order to fully secure your large amounts of BTC, the very first thing you need to secure is to keep any information about your assets private. In this way, people will not know exactly if you have many BTC stored or an alternative is to keep a ledger nano and store your funds there. If you think that keeping your seed phrase in a protected vault is not enough, the seed phrase can be memorized so that you don't need to keep a hard copy of it as well as a softcopy. The only way to steal your Bitcoins is for the intruders to compromise you. I am not fully advising this but if you hold too much BTC, diversify it with not just one ledger, split it into 5 or more. Or else, you might want to apply for a third-party cryptocurrency exchange that is regulated. If I am not mistaken, they are also storing the funds of the users in Ledger there, inside of a vault. In that way, incase your funds get compromised, they have collateral to make sure you will still get your funds.

I think multi sig is the best one, that's what I'm thinking too. Well there's no really such thing as safe since in multi signature wallet it can still be a risk you might not know two of those key holders might get your fund not unless you only require 2 signature to make transaction.