RAM upgrade what to consider?

I have a question about my RAM:

do I always have to build the same thing best so 2x 8gb RAM?

or would it also work, for example, that I take a 4 and an 8 so that I have 12 RAM or is that not compatible?

the problem is that there are 2 4s in my laptop and I would just like to buy an 8 because it is cheaper than taking out both and then buying 2 new ones. : /

Well I always buy all 4 new

You can do it, but it is not sensible. Because then dual channel is no longer available.

And that is noticeable in the speed.

Theoretically works too. It's just not that fast, but you won't really notice it.
It is important that you only install the RAM that the mainboard can do.

Yes we said otherwise I would probably also just do not want to spend too much and the question of whether it is bad to leave a 4 in it and buy an 8?

So would you rather buy 2 8s and the 2 4s out?

Can you make you hardly notice the difference