What are these tubeless accessories?

Hi all!

Following previous discussion, I decided to take the plunge (hopefully not literally) into tubeless.

A friend of mine gave me a set of valves he never used (he stayed with tubes for now), and I figured most out myself, but still I am left with 2 questions:

• what are the white soft washers? Do they go at the base of the valve inside the rim in case it is flat (instead of the current white base?)

• what are the little O-Ring washers? Are they for the valve core? The one on the valve core looks flatter.

I've deducted the black plastic things go directly on the rim, fitting it as closely as possible, and are put before the orange washer and ring.

Am I doing wrong assumptions here? What else do I know? (And what kind of sealant to buy? I've heard Stan's, is there better value for money? Intended use: MTB 2.22" and gravel 47mm)

Thank you! 🫡