Implementing a modal(lightbox) for all images in all post-gallery posts

I’m trying to make a photo gallery page for all my photos in my post-gallery post and have a modal open when you click on the photos. In the page there will probably be almost 100 photos and i’m not sure how to do this in a good way without duplicating the code for the modal 100x. I have considered using plugins but i’d doing this as part of making my own website and learning developing wordpress themes so i’d prefer if the solution is done with code.

Something Best practice solution for this situation would be great!

I would appreciate tips in how to do this!