In the game, is the SLB always on the right side of the defense ? Or is he always on the side where there is the most TEs ?

I think it's both. I think that the TE's always line up to the same side.

that is something that I have a doubt as well. because SLB means (Strong line backer) while WLB (weak line backer) and that position are according to the opposite QB arm to throw the ball

And when I go to : - My team > Depth Chart > Overrides - and when I see a defensive formation, my SLB is always LOLB in a 4-3 formation, and ROLB in a 3-4 formation ...

So maybe it's like in madden, in fact there is just ROLB and LOLB ?

In the years I played the Sam (=Strong side, Will=Weak side, Mike=MLB) always lined up over the TE if he lined up on the LOS (line of scrimmage). However there are sets where the TE lines up away from the Linemen, such as 'power'. If two TE sets are used and say a single back, its assumed that the strong side is the left (as you look from the D side of the ball).
So in short SLB = LOLB