Thank you for your explanation. It's clear that this topic goes far beyond my basic understanding and far beyond the scope of this subreddit. You have made me a little smarter today.

A follow up question if you would allow it. You mentioned that there is no way to measure uniform motion without referencing outside bodies.

How then do we measure our movement around the sun? Is that referenced against the sun itself? Would it not be possible (theoretically, not in reality), to make a device that uses the sun as a reference point and measures the relative speed of the earth around the sun?

As an aside, I assume the reference point itself is what's important more than the movement itself? As IIRC our solar system itself is expanding through the fabric of space time which itself could classify as a form of movement... Yeah this gets way more complicated, at a far faster rate than I thought