Question about Corsair RM650x *2018*

To the outside / inside ? you mean fan upwards or downwards?
Whereby it depends on the case whether you have an extra grille at the bottom so that air can get in.
The new power supply units have been mounted at the bottom for a while.
And if you have a grille at the bottom to mount the fan "outside", then I would only do it if the fan blows and doesn't suck! which it does normally. (so it sucks the air inwards normally).
Mounting it downwards / inwards depends on how high the case is, because the distance from the floor to the floor is often not high!
And if you do, you have to consider that you are sucking in dirt or crumbs from the floor!
So it's better to suck in from the inside of the PC and blow it out!
You can also hang a filter in the case fan, which is easier to clean.