How are through-hole boards mass-produced?
Mass production THT parts are either done with cheap people, or with axial insertion machines. You can find lots of videos on youtube:

https://www.youtube.com/watch?v=CCBST-qk2t8

https://www.youtube.com/watch?v=P17J5RzbsgQ

They pull the part off the bandolier, cut the leads, bend the leads, insert the part, and bend the leads on the opposite side in one stroke.