I understand that if a person uses seed from a hybrid plant that they raised and plant these, that the resulting plant will very likely not be the same as the hybrid. I would like to know how the hybrid plant seeds were produced that we buy at the store that are able to create the desired hybrid plant. Said another way, how do seed companies get the perfect hybrid seed if it is true that planting the seed from a hybrid plant won't result in getting the same hybrid?
This is a reasonably complicated thing, but put simply, the way the seed companies produce seed is to grow them exclusively, often in a greenhouse, without any other plants nearby, so that cross pollination does not occur. That's assuming the hybrids produce seed at all, because many don't, many have to be vegetatively propagated.
How hybrid seed is produced depends a lot on what type of plant is being hybridized. For example, with corn, the farmer may grow two kinds of corn in alternating rows or blocks, and remove the tassels (pollen making parts) from one corn variety so that it cannot pollinate itself, and has to rely on wind-blown pollen from the second variety nearby for pollination. In that way you would get a fairly pure hybrid seed set in the no-tassel rows. For tomatoes, I understand that the main way to hybridize them has been to do it by hand, plant by plant, by dissecting the flowers and putting the pollen of one type directly onto the pistil of another. Very intensive work. You get pure seed this way because tomatoes generally self-fertilize, so if you do that for them the chances of unwanted cross pollination is pretty low. But, again, how hybridization is done depends a lot on the type of plants being hybridized and how their flowers are constructed and how they normally pollinate when left to their own devices.
As was already mentioned, this has to be done every year by crossing the same varieties in order to continue to offer that type of hybrid seed, or the resulting hybrid plants will need to be vegetatively propagated, which these days for mass commercial production generally means cloning on a large scale in a special laboratory.