It depends on whether you're on gen1 or gen2 Cloud Run; the default execution environment is `default` which means "you have no idea because GCP selects for you" (not joking).

Counterintuitively (again, not joking): gen2 suffers from really bad startup speeds, because its more like a full-on linux VM/container than whatever weird shim environment gen1 runs. My Gen2 containers basically never start up faster than 3 seconds. Gen1 is much faster.

Note that gen1 and gen2 Cloud Run execution environments are an entirely different concept than first generation and second generation Cloud Functions. First gen Cloud Functions are their own thing. Second generation Cloud Functions can be either first generation or second generation Cloud Run workloads, because they default to the default execution environment. Believe it or not, humans made this.