why are Г read as В sometimes?

Я хочу поспать, but i'm so bothered. i've been listening to some ask reddit videos in the background and i sometimes catch the reader read Г as В and then and i don't think i've encountered it being mentioned on the basic reading and grammar rules in my references so far.

i'm so confused. is this one of those things where you just have to remember the words that read their Г as В or is there a pattern that i should be aware of?