To make commercial hybrids, yes, they maintain the parent lines, and do the cross every year to get hybrid seeds to sell.

If you like the hybrid, and save seeds from it, they'll grow into very diverse plants. If you're lucky, some of them might even resemble the hybrid you like. Save seeds from that (assuming it self-pollinated, like a tomato) and the offspring will be a little less diverse. After many generations, you might get a stable seed line that resembles the hybrid you liked.

That's only for seed-grown plants, of course. If you make a hybrid raspberry for example, you keep it going by divisions, not seeds.