Its a confusing topic! It is technically possible to open a LN channel via Core Lightning. That would enable you to send L-BTC (and assets like USDt) via Lightning.

However in most cases whats happening is that service providers are offering swaps to/from Liquid via Lightning.

This has some big benefits. Suppose for example you built up a stash of 500,000 sats on LN and you wanted to take some of it off to mainchain. Well you could "swap out" but you may be paying very high fees...for instance if you wanted to take 200,000 sats out of LN, you may pay ~5% in fees to accomplish that (depending on fees of course). Whereas using Liquid you could cash out much cheaper (ie. LN -> Liquid).

The whole idea is using Liquid sort of a "buffer" layer....between LN and BTC. This is what wallets like AQUA are doing for you automatically.