Just saw the Liverpool vs Manchester United game, and I got confused on the offside rule when Salah scored a goal. Salah was well offside when Allison passed the ball to him. I'm confused that how is that not an offside I thought it won't be offside when goalkeeper passes the ball only when its a Goal Kick but it wasn't a goal kick Allison just got the ball after deflecting from one of the man.u player. Just need some clarification on this. Thank you

While watching the Liverpool vs Manchester United Premier League game in January 2020, I got confused on the offside rule when Salah scored a goal. 

Salah was well offside when Allison passed the ball to him. I'm confused as to how that is not an offside. I thought it won't be offside when goalkeeper passes the ball only when it's a goal kick, but it wasn't a goal kick, Allison got the ball in open play after deflecting from one of the Manchester players.