Just as an addition to what fatfork already said about companies preferences in selecting which wallet that best meat their preference at a time, but primarily most companies will only use hot wallet for transactions but most definitely since most of their daily transactions will be in fiat so bitcoin won't be a big option for them so at that milti sig cold wallet is their best shot, just as fat fork said this is just an assumptions since most companies won't reveal such info.



The image above shows how/ways businesses handles their Bitcoin reserves.
Source: https://river.com/learn/how-should-a-business-store-bitcoin/

Multisig, large custodians like Coinbase specifically meant for big companies such as Microstrategy and the likes of them.

This is a question that even if Michael Saylor is asked, there will not tell you as if will possibly compromise their security.

But everything that Fatfork has answered correct.

Maybe a bit off topic here but I was reading something on Reddit the other day that said the feds apparently found the seed phrase or whatever for one of FTX's cold wallets or something that held tens of millions of dollars.  It was just written down on a scrap of paper sitting on someone's desk.  Like next to a bunch of other random papers and stuff and  they didnt even remember they had the wallet until the investigators were searching their offices.  If thats true, it really shows how little some of these companies cared about properly securing their assets and accounts.  Though I guess we already knew FTX was shady and not a serious company.  

For me there is no big deal on where company needs to store there bitcoin, they can store in a any non custodial wallet, provided the seed phrase is safe and  secured and also not accessible to anyone apart from the the person in charge of the company finance. And also the device used for storing of that wallet should be strongly protected by malware protection and antivirus to protect it from phishing attack or viruses.  Apart from that the may decide to store in a cold wallet and many other.

Why do you need to know where they store their Bitcoin and how they do it? There is something called privacy, this is what Bitcoin gives you that nobody can interfere with your investment unlike the banks where even the government can have access to the account number and the names of the account. The companies store Bitcoin just the way you store your own Bitcoin, they used hardware wallet like the ones you used like Trezor and ledger.

There way of storing Bitcoin on their wallet might not be straight forward like a single spender, they do create wallet with M of N signature where 2 or more private keys are required before any Bitcoin can be spend from the wallet. This protect the compby from the fund been mismanage by one person or stolen by some. Even in a situation when one person who has the ledger is been compromised, the funds can be spent because of the M of N signature requirements.

Nowadays, storing bitcoin in single signature wallet is no longer popular especially if it is a company, corporation with big fund. They will use multisig wallets for storage, cold wallets but there are entities that don't want to store bitcoins by themselves, but choose custodians for example Coinbase doing this for them.

I don't say using a third party service, like Coinbase, as a custodian for your Bitcoin fund, is safe and best way of storage, but it's fact. You can check it with Bitcoin Spot ETFs in the USA and many of them choose Coinbase as their custodians.

Reminder: do not keep your money in online accounts
Bitcoin Spot ETF Custodians.

Multisig wallet with co-signers can help to increase security of fund from wallet hack or abusement of a single person. With a single signature wallet, if a single person dies or if he abuses it, the fund will be stolen.

It will be the end for that company if the founder steals the company fund and run away, or if the founder and the only wallet owner died without any back up left for anyone else.

A Crypto Exchange CEO Dies—With the Only Key to $137 Million.

when it comes to this, multisig always appear in my mind, no way the company will just let one person oversee their wallet and if something bad happen all the asset gone.

but knowing that managing the private key on their own gonna add some unnecessary risk and point of failure, i'd bet they will let other company that specializes in crypto and is highly trustable to be managing their bitcoin.

but yeah, there's always possibility that they are creating their own tools, just for the sake of security but I doubt most of big companies are doing that, and also, they definitely won't tell you about how they secured their bitcoin.

As I know big companies usually store their bitcoins with big quantities in something called cold wallets (These are wallets not connected to the internet ) which makes them very secure against hacking. They might use hardware wallets like Ledger or Trezor or even create custom solutions in strong and secure computers. For extra security, they often use multi-signature (multi-sig) wallets where several people need to approve a transaction before funds can move, and thus is in case someone could get access to that computer or hacked it.
Accessing the wallet needs a procedure with private keys. In multi-sig setups many wallet should be combined and no single person has complete control on the funds. Companies also back up the keys in secure locations,
like vaults.

Honestly, I find that half or even the more important part on storing bitcoins is not just about the method or place but also avoiding many mistakes to let hackers easily get access to you and make the company vulnerable .