It's exactly what you're seeing in the videos: the fretting hand and the plucking/picking hand work together to tap out notes - pressing hard enough on the fingerboard, obviously.
Basically, if you watch carefully, the musician will sound a note by tapping (and holding) with a fretting hand/finger; then as that note is usually being held, the other hand/finger will hit a second note that is appropriate for whatever is being played. Lather. Rinse. Repeat. There's no secret formula - the player kind of needs to know what they're playing to make it sound good. Knowing your fingerboard, scales and etc, helps immensely - just like every exercise/technique on the guitar. Practice!
I just quickly searched YouTube (don't forget Google): "how to tap bass." There seem to be lots of hits. Try some of them.