Its a bit too late for my cat but I want to know what happened so I can avoid it in the future

I got home from summer school and found my mom's cat in the basement (which she almost always avoids) laying down having trouble breathing, her gums were kinda yellowish-white and she was too weak to even stand, my mother and grandma have both been seeing her in secluded spots so I did my best to comfort her in our basement while my mom drove home, it became incredibly clear she was going to die (which she unfortunately did) in my arms. I picked her up (which she usually hates) but this time she purred real loud, looked at me, meowed softly, licked my arm and closed her eyes and that was it. I'm sorry if this isn't the right sub im trying to find answers. I'll provide a video if requested, but beware she is not in a good condition in said video