actually can someone explain me the difference of each chain? i'm very confused, each NFT say me to change network and i just do it but i don't know what's it
ERC20 is the protocol of some sort, specifically for Ethereum based tokens, that is, all tokens found on the Ethereum blockchain are classified as ERC20 tokens. On the other hand, BSC is that of Binance which also means that, all tokens based on BSC network are called BEP20 tokens.
Now the interesting part is, most blockchain networks today, are all copying many features from the Ethereum blockchain, hence the reason why your Ethereum wallet address, can also be used to receive BEP20 tokens, with all that is needed being to change the network from Ethereum to BSC.
This is exactly the reason why you were asked to change network in order to view your tokens which can easily be done by adding the BSC network on Metamask and view your tokens.
Also, nowadays many mobile applications like Tokenpocket now makes it easier, that is, by creating a wallet on Tokenpocket, saving the backup words, you can effortlessly switch from Ethereum network to BSC network. So when you receive a token, irrespective of the network it is based on, you can easily see it in your wallet, unless you will need the contract address to add the tokens in most cases.