What is a layer 2 exactly?

Hello,

So polygon is a sidechain, a layer 2, you can transfert your asset on polygon and do some dex trading and defi stuff over there for less gaz fees and then transfert back to ethereum if you want to.

But here is the thing : not all eth token exists on polygon, from what I understand the token owner has to create the same token on polygon then use the "project mapper" to says "hey it's the same"

So, why are not every EVM compatible chain called Layer 2? I can create a token on both ETH and BSC and they make a bridge. and record in coinmarketcap every token contract adresss for all my tokens on all those EVM compatible chain, achieving the same result.

Thanks.