How would you find out which standard a certain cryptocurrency is, that is whether the crypto is ERC-20, BEP-20 or something else?
ERC-20, etc. are not cryptocurrencies, they are tokens. To know what type of token you are dealing with you have to check out the website of that token or its announcement on bitcointalk.
Can a crypto belong to only one network? Where can you check network affinity? Must a crypto have different contract addresses in different networks? How can you check which crypto is the native token in a given network?
Any cryptocurrency (or a token) can only exist in a single blockchain and nowhere else.
However there are ways we can defined a secondary "network" where the same coin can exist in. For example bitcoin has only one blockchain and only exists there and nowhere else but it can have a second layer (such as lightning network) and same bitcoin can be used in that network.
This should not be confused with shitcoins that abuse the name of a popular coin. For example there are tokens available on shitcoin such as Ethereum that use the same name. These tokens have nothing to do with bitcoin even if they promise 1:1 swap rate and use the same name.