As another not-physicist who is interested in physics, I've found the articles/explainers at Of Particular Significance very helpful. There are a few on particle decay, and I think that these two provide a longer answer to your questions:
Most Particles Decay — But Why?
https://profmattstrassler.com/articles-and-posts/particle-ph...
Most Particles Decay — Yet Some Don’t!
https://profmattstrassler.com/articles-and-posts/particle-ph...
Neutron Stability in Atomic Nuclei
https://profmattstrassler.com/articles-and-posts/particle-ph...