I have done some soldering over the course of my relearning how to make, but I didn’t do so deliberately or with really decent instruction.
I found a really fantastic video series on soldering, which looks like it was from the 60s or so:
Covers all the basics, and I learned a fair amount from it. It is a super simple explanation of what is actually happening, and how to get a decent solder joint. Probably the best and simplest video about the fundamentals that I have seen!