What is going on and is faster / better?

Ni
1

I wanted to know what you would recommend me.

I was looking for a laptop, had found an interesting model and now noticed that you can either take it with 2 x 4 GB RAM (once onboard, once selectable) or with 1 x 16 GB RAM (selectable, but then without onboard).

First of all: why is that?

But more important to me: How much power would be lost if I only had one RAM… (whatever you call it) instead of 2?

Gr

This is irrelevant for you as a "user" and noticeably hardly relevant.
in any case, a higher total working memory would be preferable because the storage would be larger here.
It would be more important if an SSD was installed - you can really notice this effectively at boot time and also when accessing work.