How L2 actually scale Ethereum. Can’t get my head around it

Read alot about L2 solutions for Ethereum, but couldnt find an answer to that question. Where and how the scaling is actually happening ??

one of the things I was curious about L2 scaling solution is that some of thing look like are built to increase ETH capacity, but in reality they are working on their own mainnet (network) which doesn't look like an actual scaling.

I guess some L2s are actual L2s but then some are labelled L2 without actual contribution to Ethereuem blockchain scalability

Thanks