About Base Font and REM units

Hello,
I would like to get some clarification because I think that over the years I became confused, or at least I didn’t pay enough attention to this topic as I should have. :)

I want the fonts on my website to be responsive. For that purpose, I intend to use the "rem" unit.

I thought that by setting the "Base Font" to 16px in the theme settings, I could then define the paragraph as 1rem, H1 as 1.5rem, and so on.
However, it seems that there is no association between the Base Font size and the other styles.
(I read this somewhere in a forum post and it appears to be happening, apparently.)

My questions are:

In practice, what is the Base Font used for?
How should I use the "rem" unit?

1 – Should it be used only in plugins (for example, a text plugin)?
2 – Or can it be used in "Theme settings → Typography"?

In the latter case, the rem unit seems to have no size reference to relate to…
I’m not sure if I explained myself well.

Thank you, and I apologize for this question / doubt.

Nuno F.

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