This is a 2-of-3 multisig wallet. It is worth knowing that you can generate more keys and more cosigners up to 15.

Altcoins can have multisig, but I do not know if they can be for a a wallet that is supporting multiple coins. The scam in which a scammer will tell a stranger that he has USDT on Tron blockchain and need the help of another person to withdraw the coin, the scammer used a multisig wallet to make the scam possible.

Ethereum, solana and many other coins support multisig.

Your family member is only partly correct, and it is true that in Bitcoin, rules for multisig are defined in script but multisig is by no means Bitcoin only. On Ethereum, for example, there are multisig options (eg. Safe), although rather than being defined in script, it is more of a smart contract wallet situation. On Solana you will get other solutions again. So while not common everywhere, you CAN set up multisig for various blockchains.

Whether or not you can use the same signers depends on whether they are supported both by the software wallet and the signer(s). Not many pieces of software are going to support multisig for multiple coins, and you may very well find you need a wallet for each, eg. Sparrow for Bitcoin, Safe for Ethereum, etc.

And lastly, seeds are not everything in Bitcoin, especially with multisig. You need to save the descriptor or related xpubs as well as the seeds in order to recover it.

In every transaction, to send coins you'll need to create a smart contract using the Bitcoin's script language. It is like a locking mechanism where you "lock" the coins up inside an output script and in order to spend those coins the receiver has to provide the "unlocking script".

These scripts can be anything from a simple mathematic problem like "add 2 numbers and if their sum was equal to a third number, evaluate to true):
which is obviously something anybody can spend; to the most common script which is "duplicate the top stack item, hash it and verify its equality to a given hash, then perform a single signature verification, if it were valid then evaulate to true":
which is what you have seen for over a decade as P2PKH addresses (the ones that start with 1).

Multi-sig is just another "locking" script you are creating that has a different conditions to be met that includes multiple signature verification against the given public keys. How many? It depends on the script but it can be up to 20.

It depends on the smart contract capabilities of those other coins. Many of them copy bitcoin's so they too support similar multi-sig scripts to be made.

See the scripts in pooyah's examples?
Those are what the address are when decoded back into scripts, a.k.a. "scriptPubKey", you could say "rule(s)" on how to spend it.

Most probably what your family member meant by saying "built into Bitcoin address" since addresses represent a script underneath.

I'd say he's right (in a way, it's too oversimplified though),
But you misunderstood it because the used term is "Bitcoin address", so you thought that it's limited to Bitcoin.

Since address is just an encoding of a script, if an Altcoin supports such script (e.g.: Bitcoin clones like Bitcoin Forks, Litecoin, Dogecoin), you can setup the same MultiSig wallet type for those coins.
Of course, it'll result with different addresses per coin not only because the private keys are derived with a specific derivation path per coin, but also, different address encoding per coin.
And other Altcoins use their own MultiSig methods as other users explained.

You can set up multisig wallets for many cryptographic coins other than bitcoin. The doubt you may have developed that multisig functionality is limited to bitcoin is incorrect. Multisig can be successfully implemented for various other coins, either at the protocol level or through smart contracts. In Ethereum, while there isn't a native multisig address script coded exactly like bitcoin’s, multisig functionality is highly popular and robustly implemented via smart contracts. Furthermore, almost all major modern blockchains such as Solana, Cardano, Polkadot, and Litecoin offer the capability to create multisig wallets through their respective protocols or smart contracts.

Both numbers are correct.

The “up to 15 keys” number is from max script element size. Redeem script (which is the case with a P2SH multisig) is limited to 520 bytes. Factoring in the overhead of the multisig script and the size of each (compressed) pubkey, this works out to 15 keys max.

The “up to 20 keys” number is the actual limit on OP_CHECKMULTISIG (MAX_PUBKEYS_PER_MULTISIG).

Note that with SegWit this changes a bit: with P2WSH (native SegWit) multisigs you’re not limited to 520 bytes, but you’re still limited to 20 keys with OP_CHECKMULTISIG.

Anyway, seems like a good distinction to know if you are planning on doing any nontypical multisig. Hope this helps OP.

Creating wallet that carries multisig on trust wallet comprises of Bitcoin and other altcoins with only 1 set of seed phrase, so I don't know the wallet your friend is talking about?

If you want to only create a Bitcoin only wallet on trust wallet, you simply not going to choose multisig option, if you want want other altcoins to be included, then multisig option you need.

Imo.

One important thing to remember is that multisig security also depends on how you manage your keys and recovery setup. Even if a blockchain supports multisig , you still need to make sure your wallet software supports it properly. I’d also recommend testing the recovery process with a small amount before storing significant funds.