https://news.ycombinator.com/item?id=39711815 Shopify seems to have millions of customers. Granted, they have MySQL and it might not use database schemas and something more MySQL specific. In general though ...
Sounds like this is just normal sharding. Completely different from having a totally separate schema like GP proposes (with that approach you don't use sharding keys at all).
But I do see now, searching for that part of the info specifically, they mention
Peppy Peppers’ orders can be migrated by running SELECT FROM orders WHERE shop_id = 1 FOR UPDATE on Pod 1’s database