What is the cheapest way to buy ether and use for basic defi like staking on lido?

I want to stack ether and put it to use but ideally in a simple method such as staking.

I am curious if someone can explain if there is a known cheapest way to buy ether and use it in defi?

Previously I bought ether for cheap on loopring L2 it needs to be brought to L1 to use in other apps.

Can ether be bought directly on optimism and polygon and interact with dapps from there?

I'm trying to grasp all this so thanks for the help and sorry if I'm completely misunderstanding something.