so shards have a cap of 200 players or had soft cap of 200 (you could raid form to distort it) and the 200 soft cap was from BFA maybe different in SL.
Blizzard decides what ratio of players Horde/Alliance get on a shard, originally it was unbiased but an early patch in BFA changed this around nov/dec of 2018 to make it a 7:3 split for Ally:horde. So if there are 420 ally and 610+ Horde you will get the resulting shard pops:
Shard 1
140 ally vs 70 Horde
Shard 2
140 ally vs 70 Horde
Shard 3
140 ally vs 70 Horde
Shard 4 (no more alliance)
200 Horde
Shard 5 (no more alliance)
200 Horde
This is what makes it seem Ally are everywhere because if you see 1 that generally means theres like another 14 right there loading in, and some Horde will be lucky/unlucky enough to be on “dead” shards with 0 alliance at peak times.
When alliance form raids to flood a shard and push it well past its soft cap of 200 (think all premade Nazjatar events) the Shard/server falls apart… and also destroys alliance parity on remaining shards.