Confused about old xpac scaling

Hi, so since all the expansion zones are for the 10-50 levels, how does legacy bonuses work for soloing old content? Also I’ve noticed on some of my characters I’m still doing other expansion content without having gone through Chromie time. For example, on this toon I’m doing WOD levelling and I’ve not once talked to Chromie, but stuff still seems to scale correctly. Although my dungeon finder only lets me queue BFA dungeons.

I’ve heard some people say you can travel between different expansion content freely. Do the rewards scale up to and including level 50, or does some legacy threshold kick in at 50 so rewards stop scaling up? I’m wondering if it would make sense just for the prepatch to start Legion at level 50, as I expect the artifact weapon would have a decent ilvl for level 50?