Gallery functionality in custom theme

I'm new to creating my own theme and I barely scratched the surface of this topic so far. But my end goal would be to create a gallery like functionality.

Maybe there are some functions or prebuild things in WP that could help me out here before I do all of this from scratch.

Some terms what I can google for or look in the documentation would help me out a lot. Because when I search for stuff on google I only find some basic things like finished themes, plugins etc. but no development info (besides the docs).

As I said, I would like to have some sort of gallery where the user in the backend has a button to add an image (in the best case) a pop up (or other page) will open where the user enters some information (image from media uploader, image name, tags etc.).

The tricky part for me is that I want to have X amount of images possible. So when one is added the add image button is still there and the user can add as much as he wants.

So I don't define that only (for example) 10 images are possible in 10 hardcoded fields.

If that pop up thingy is too complicated/ not existing in WP (in an easy way) it would be enough to have new fields generated automatically. So the user has a button Add Image - On click three fields appear - The user fills the fields - Add Image button again - Another three fields are generated and the user can also fill them.

If you need more information, please let me know.

I'm glad for any help!

On reddit somebody suggested the use of Advanced Custom Fields. But I would like to have a solution without the use of third party plugins, especially paid ones.