HP Pavilion 15-ec0315ng notebook open the back?

Unfortunately I don't know how the back opens to change the RAM, all screws have been unscrewed.

Hidden screws?

Can't find a tutorial for the notebook either, maybe someone from you has the same?

See if that helps:

https://blog.notebooksbilliger.de/praxistipp-hp-pavilion-15-in-10-minuten-erweitern-mit-step-by-step-anleitung-und-video/

As a rule, the bottom D cover can be opened in almost all notebooks.

You remove all the screws and really make sure that you got them all.
You take a plastic spatula (or similar, but nothing metallic) and use it to reach between the C-cover and D-cover and pry the D-cover out.
Make sure that you don't bend any interfaces or lever them in too deeply or incorrectly. It is really important that you carefully lever out the tabs of the bracket, this makes several light claws.

The covers are composed as follows:
A-Cover: back of the display
B-Cover: The bezel around the display
C-Cover: The cover of your keyboard
D-Cover: The underside of your notebook

Ohh yes. Lots of hidden screws and even then you still have to pry open the gap. Here you can find instructions for the entire HP Pavilion 15 series.

Do you happen to know whether there's a screw under the glued rubber?

Watch here:

Yes it is, and don't forget to pull out the DVD drive, there are also 2 screws.

Ok thank you then I'll remove that too

BATTERY out? There are screws there too.

I didn't remove it because I didn't find any screws there either

Aren't they hidden under the AKKU bridge?

As far as I can see, the notebook doesn't have a battery bridge… I've already found something