How can small ASW vessels find a Submarine?

This short 6 minute, September 2020, "sub brief" Youtube at https://youtu.be/LWM259XA19I and below:

concerns the 1,000 ton Grisha-class ASW corvettes built by the Soviet Union, 1970-1990, and then by Russia and Ukraine. The American briefer was an electronic warfare submariner up to the 2000s.

If only the Grishas had the extra tonnage to carry ASW helicopters their tactics would have been far more effective in hunting "free world" submarines. See Submarine Matters' later post Soviet ASW Helicopter Tactics, of October 16, 2020.