I spent a bit of time as a commercial plant breeder. Most of the stuff you find online is inaccurate. Genetics and plant breeding is extremely complicated and species specific. It's not something the average master gardener even has the fundamentals to understand.

For example the answer to your question about saving hybrid corn seed.

One plant of a F1 commercial contains the same heterogenity as a healthy 1,000 plant OP variety population. Commercial hybrids are created by crossing inbreds from separate genetic pools. This helps maintain the required genetic distance between the inbreds to maximize heterosis.

1,000 F2 plants from one F1 is enough to initiate a stable population. Saving less than 1,000 plants on the subsequent generations will create a bottleneck and the population will quickly suffer from inbred depression.