Dawniest, you might not be an electrician "officially", but you already revealed you will tinker in it.
If your main panel is full, you can pick a couple of breakers in it and remove them. That frees up a space to put a new breaker that will actually serve as the breaker for a sub-panel. The subpanel serves the purpose of making room for having more breakers. If it is installed really close to the current panel, it really is just an expansion of breaker space.
Example: Yank a couple of 20amp breakers out. Replace them with a 60amp. Wire that breaker to power the subpanel. (using appropriate wire size)
Now that sweet new panel is a 60 amp service with a bunch of room in it. The first breakers to use this panel are the 2 you removed from the main panel.
All of this is "conceptual" only. I'm not an electrician either. Don't listen to me.
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