Battle Tree (except for battles on the normal tracks in USUM) automatically scales your team down to level 50 if it's higher than that, and recalculates the stats to match that level. (If your levels are lower than that, they're allowed to stay as they are, in case you want to utilize strategies involving a level 1 Togedemaru or something.) All opponents, regardless of track, will be scaled to 50 just the same, resulting in Pokemon that are perfectly allowed at 50 even when their natural evolution level is higher, such as Braviary, Tyranitar, or Hydreigon.
Once you unlock the super tracks, you can bring your own high-level species like those and see that they're adjusted down to 50 just like they were. You can also bring Tapus and Ultra Beasts, despite those being caught at level 60.
