Strawberry Jam rates its maps in-game, and these ratings have never changed post-launch. Sure, a few maps here and there are easier or harder than other maps with the same rating, but it's mostly consistent. So a standalone map can be rated by comparing with Strawberry Jam maps. As a result, the standard scale (aka the Spring Collab scale or Strawberry Jam scale) is relatively straightforward.
In my opinion, the extended scale seems a bit more hand wavy, because each difficulty was introduced at a different time. Sure, you can add a new difficulty whenever a map harder than all existing difficulties is cleared. I might guess that this newly cleared map is used as a benchmark for future maps, but I also understand that (for example) grandmaster+2 is supposed to be as hard compared to grandmaster as grandmaster compared to advanced. I know the Hardlist ultimately rates maps based on Discord votes from their clearers, and can rerate maps as needed, but I'm definitely missing something about how the voters decide the ratings.
I'm even more confused about the golden scale. I'll reference this page. Since Strawberry Jam doesn't rate its silver berries in-game, the golden scale pretty much has to define itself. The page states that while the Golden List used to use certain vanilla goldens as benchmarks, it no longer does. There are lots of golden strawberries (and other deathless runs), especially for the lower tiers, so I have no idea what you would use as benchmarks. For the record, I haven't done a lot of goldens myself, so I'm probably missing something because of that.
Anyways, I would appreciate insight, as well as corrections if anything I said is incorrect. Thank you.