There are many ERC20 tokens/coins
If you know an ethereum account number, e.g. 0x6D6E8411B0507dA903D1E9f32E6cc42951F3C17f (pulled at random from etherscan)
Can this account be used to transfer/hold multiple coins? E.g. Ethereum and TRX and BNB etc?
I.e. when you look at the transaction history for an account, you cant tell what coin the transaction is for (many transactions haven a value of 0 Ether, presumably because they are transferring something other than ether)?
Yes, an account can own multiple ERC-20 tokens as well as Ether.
Transactions don't always mean that an account is sending assets. Calling a function in smart contract that alters the state is also done with transactions.
Just to clarify: there are no ERC20 coins, only ERC20 tokens. Ether is the only coin that can be held in an Ethereum account.
For more about the difference, check for example https://blog.citowise.com/the-basics-coin-vs-token-what-is-the-difference-5cd270591538
