These things may have been already said:
1)Peppers and squashes often have mature seeds in them Tomatoes, many veg you buy at the grocery store has viable seeds in it.
When I core a bell pepper sometimes I dry the core(the stem area with the seeds) on top of my refrigerator.
When I was growing "pickling" cucumbers they grew way bigger than regular cucumbers and the skin went yellow and they got really big and bitter.
After TASTNG THE OLD ONES.
I put them on the compost pile.
And pickling cucumber vines grew.
Now I have a berm I put things on/in I don't want to use right now in the food forest.
Most of the veg you buy at the grocery store has potential to have viable seeds.
The veg you grow, always let some pants live the entire life cycle.
If you miss out on the seeds they make, if you remember where they were......you can propagate by spreading the dirt.