That might be more a history or archeology question than a science question.
Early domestication
While there was no concept of long term evolution, many people could witness that offspring of individuals (whether plants or animals, incl. humans) looked like their parents. From theirthere, it is not much of a stretch to understand that by breeding individuals with desired traits, you would get offspring with desired traits. This is artificial selection at play.
Artificial insemination
Early animal domestication did not use much artificial insemination. According to Britannica.com, the first successful artificial insemination in animal was in 1780s only.
Intended vs unintended selection pressures
Note however that I do not want to claim that most of the domestication traits evolve by the human intention to make them more frequent in the population. It might be just that individuals poorly adapted to domestication would flee or get harmed or something and would therefore have lower survival. For example, aggressive individuals could get hurt by their farmer.
Note btw, that wether the selection pressure by the farmers were intended or not, we still typically use the term artificial selection.