I have grown tomatoes from seeds I saved from my own tomatoes. Seed Savers Exchange has lots (lots-lots-lots) of info about growing, harvesting, and saving seeds for lots of fruit and vegetables. Here's some info about tomatoes:
https://www.seedsavers.org/grow-tomato
I would trust any info I got on this site to be legit. It might not work for me/my climate, but it is good info.
For any kind of plant, a hybrid is a mix of more than one variety. Just like children are a mix of their parents' genes.
If you want to save seeds, look for fruit/veg that are labeled "heirloom." They will "breed true" and their seeds should produce plants that will bear fruit like themselves.
I don't personally know of any tree fruit you would buy in a store that would reproduce the same variety from its seeds. If you want to grow fruit trees, buy a tree from a reputable nursery. Expect to wait a few to several years before the tree bears any fruit. We planted some sour cherry trees (Montmorency variety) 2 years ago. Last year, they had a couple of flowers, no fruit. This year, they had lots of flowers, but still no fruit. They might bear some fruit next year. They just need to be old enough.
Another note about fruit trees: Many need another variety for cross pollination, or they won't bear fruit. Sweet cherries must have another variety. Sour cherries don't. Most, maybe all, apples do. If you're buying trees, they should have this info with the tree, but ask anyway.
