For small-scale migrations (a handful of shared drives and groups) where standing up Google Takeout / the Data Migration Service isn't worth the overhead. Does not cover individual users' data; see Scope below.
Generic, replicable procedure for pulling a custom domain out of one Google Workspace (or Cloud Identity) tenant and re-registering it in another, when you are superadmin of both. Written from a real migration; examples use placeholder domains so it can be followed for any pair of tenants.