Probably 99% of produce is from hybrid plants, and seeds saved will not breed true. They will grow and probably will produce some sort of harvest, but instead of nice beefsteak tomatoes you may get tiny, sour cherry type tomatoes... it just depends on what genes are expressed.
IF you have lots of time, energy and space, it can be fun to experiment. Even organic growers use mostly hybrids, but if you can find varieties advertised as "heirloom", they shouod be an open pollinated variety, but even they may have been crossed during the growing season.
You can get lucky... years ago, an old family friend brought us sone seeds from a butternut squash he'd bought at the farmers market. To be polite (because I figured it had to be a hybrid) I grew them, and had a great crop of large, uniform, tasty butternut squashes! I've saved seed from them a couple of times, and need to grow it out again this year to freshen my supply.
And there are ways to "breed back" a hybrid to stabilize it as an OP variety, although it takes years.
One place where my advice tends to clash with how most think is that I recommend that at least some of your seed supply should be in reliable hybrids. No, you won't be able to save seeds (true to type) from them, but when talking survival, take one step at a time. First, you have to survive long enough to harvest! Hybrids tend to be "designed" with various desirable traits bred in... resistance to various diseases, high yields, longer production season. The introduction of hybrid seed corn (long before GMOs) DOUBLED the grain yield in a few short years.
So, if you stash some reliable hybrids that grow in your area, at least you'll have food to eat while you are developing your open pollinated varieties and learning how to maximize production from them.
Summerthyme