I remember feeling a similar confusion. They are both Bitcoin but you can only only send from one to the other in certain circumstances.

Here's why:

Bitcoin transactions are recorded in blocks on Bitcoin's blockchain.

Lightning transactions are a way of making an agreement with another person that you'll each keep track of the transactions you make with each other and only settle the net balance in a transaction on the blockchain every once in a while.

So bitcoin on lightning and bitcoin on the mainchain are both bitcoin.

Where it gets weird is if you have bitcoin in a lightning wallet you can't simply send it to an on chain wallet. This is because your bitcoin on lightning needs to net out with your channel partner before you move some of it on chain. There are lots of ways of dealing with this (swaps, splicing, closing your channel), but the main thing to remember is that bitcoin on lightning involves an agreement with another person and so you need am extra step or two before you can move your sats on the mainchain.

Many modern lightning wallets will let you pay an on chain address and will handle everything in the background.