What is shared
Address derivation, so one key gives you the same address on Ethereum, BNB Chain, Polygon, Arbitrum, Base and every other EVM network. Transaction format, so one wallet speaks to all of them. Contract bytecode, so a protocol can deploy nearly unchanged code to each.
Token standards too, which is why ERC-20 behaviour is predictable across chains.
What is emphatically not shared
State. Your balance on one chain does not exist on another. A token deployed on Ethereum has no presence on Polygon unless someone deployed it there, and that deployment is a different contract at a different address.
Chain ID separates them at the protocol level, which is what stops a transaction signed for one network being replayed on another.
The mistake this enables
Because the address is identical everywhere, a transfer to the right address on the wrong network is accepted and recorded. The funds now sit on a chain where you may or may not control that address — and if the destination is an exchange or a contract, often you do not.
Recovery depends entirely on whether whoever controls that address on that chain can and will act.
Working safely across chains
Confirm the network on both sides of every transfer. Add tokens by contract address for the specific chain, never by symbol. Keep a small native-coin balance on each chain you use so you are never stuck.
FBT Swap supports sixteen EVM networks plus Solana and displays the selected network before every signature, which is the one place this error can still be caught.