Gear loses oil (with photos)

Hello guys, My Seat Ibiza 6k, year 97 loses oil. Probably on the gearbox. I took 2 pictures of it, maybe you can see something on it. Unfortunately I don’t know so much, but I don’t want to go to a workshop, if you can still do it yourself. Therefore the question to you, what can the problem be?! At the rear part of the gearbox (red lines picture 2) everything has accumulated. With the Simmerrings everything seemed to be dry (i.e. picture 1), can of course But on the other hand there is a large inner hexagon screw (red circled in picture 2), which was a bit oily… but it was stuck and actually nothing can come from it, can’t it? Where could I still look? With regards to Maccron