owners vs site owners vs members vs site members vs access to site content

I'm fairly new to SharePoint administrations and have been trying to wrap my head around permissions...

Can someone explain to me the difference between owners and site owners as in the screenshot below, and how it relates to access to the site?

I found this article (https://learn.microsoft.com/en-US/sharepoint/site-permissions?WT.mc_id=365AdminCSH_spo) but there must be something more to it.

I've removed someone from the "Owners" group but kept this same person in three other groups where she was present: "Site owners", "Members" and "Site Members".

After I removed this person from the "Owners" group, this person lost access to a document library folder. The document library in question has restricted access with a number of people being granted access nominally. I don't know if this person was in the list or if, being an "Owner" and "Site owner", the access was granted automatically.

Nevertheless, re-introducing this person as an "Owner" was not enough and I had to grant her access nominally to that restricted document library.

So I don't understand why (1) removing from "Owner" (even kepping her in all other groups) made her lose access and (2) why I had to grant access nominally (being an "Owner" and "Site owner" was not enough)?

Any ideas?

Thanks,

Alex