Producing hybrid corn seeds, for instance, is done by planting the two varieties in the same field, with one combine-width of rows of variety A alternating with rows of variety B. Then you need to wait until variety A is grown and ready to tassel, and de-tassel it before those tassels fully form and release the pollen that drifts onto the corn silk. This is usually done by hand, by a bunch of people walking through the field and cutting the stalk between the ears and the tassels.
Once that is done, your variety A ears will be pollinated by variety B pollen, and all of those kernels will be hybrid. But your variety B ears be purebred B, not hybrid, so you now you need to harvest the field while keeping the two sets separate. This step isn't too bad, if you did a good job planting, you just need to first harvest the combine-wide strips of hybrid corn before going back to get the B corn.
But yeah, you're looking at a lot of extra work -- and money, you need contract labor to detassel a field in the narrow window of opportunity -- to produce a hybrid seed for better yields next year, and most farmers don't find it a good use of time, which is why most farmers were buying most of their seed well before seed patents and GMO were a thing.