My mom used to save most seeds from her plants. Some seeds like radishes, and lettuce, she purchased, because it was easier, than letting plant go to seed.
She also acquired a few varieties of tomato seeds from tomatoes she purchased from grocery store. It tomato looked good

, and tasted delicious



,,,,,,,,,,,,, she kept seeds from it for planting.
On her tomato plants, after a few season cycles, the tomato genetics changed, most likely due to bees cross pollinating with other tomato varieties, she did same thing over.
I'm not a BOTANIST,

and do not play one on TV,

, but I think the seed producers keep their different varieties separated, to prevent cross pollination.