Please can somebody explain to me how L2s such as Polygon actually work?

I understand how it helps scale Ethereum by taking traffic off the main chain and onto the side chain, but how is the side chain actually connected to Ethereum?

The are tokens that are bridged onto the Polygon network wrapped tokens or are they the “original” ERC20 tokens? Also how to L2s use the “security” of the Ethereum chain even though they’re done on the side chain?

Any help would be greatly appreciated