I just can't find a single scenario where Mak's Ambush is applicable. If he can see another unit then the other unit can see him. Could somebody please explain me how ambush actually works? A diagram would be great, but not necessary.

When he's healthy, Ambush can be triggered with his 2nd ability, the one that says hostile figures 4 or more spaces away cannot see him

When he's wounded then yes Ambush gets much harder to trigger but it's still possible. For example see Rules Reference Guide (RRG) page 26, the appendix, it shows you examples when figure A can see figure B but not vice versa

If he has Target Acquired it's also easier to trigger Ambush, it just costs an extra strain and there has to be another figure in between him and target.

Covert allows Mak to be more than 3 spaces away and the target doesn't have line of sight back.

There are a lot of cases where there is no reciprocal line of sight, when a figure attacks from between diagonal intersection of two figures or one figure and wall/door/blocking terrain. See Luke in example (17). He has line of sight to the Stormtrooper, while Stormtrooper doesn't have line of sight back.

Also see:

https://boardgamegeek.com/thread/1684965/maks-ambush-ability

That explains a lot, thank you!

His reward from his personal mission also assists greatly in this regard.