I disagree with this. When I was first starting out, building power supplies was a good exercise for me and produced something actually useful. I still have a 5V 7805 power supply and a ±12V power supply made from zeners as pass transistors that still work 40 years later. ...
When I started out, about 1950 or so, I had to beg money from my mom or my grandmother for dry cell batteries to do simple electrical experiments. There was little opportunity for a six year old child to earn money by working. But I soon "graduated" to building power supplies from salvaged transformers and selenium rectifier stacks. It was ten years later before I could afford the new-fangled silicon diodes and silicon controlled rectifiers. Still later, while in my late teens, I built a variable voltage-regulated vacuum tube power supply, still using salvaged parts (mostly from TVs by then), for my first amateur radio transmitter as well as for bench use. That was the last time I did any serious work with vacuum tube electronics design for almost another twenty years. There were many power supply projects in between.
Did I learn anything from constructing power supplies? You bet I did. I also learned that if I could afford to purchase one, I could get on with my electronics hobby much easier and quicker! Hence my recommendation to beginners: there will be plenty of opportunities to build power supplies, and you definitely need to learn how to do that, but if you want to learn electronics you need a reliable, stable, power supply
right now. Purchase a good one now and you will use it forever. Learn later how to build custom power supplies for your projects. Well, maybe not too much later... sooner is better, but get the basics down pat first. Ohm's Law, Kirchoff's Laws, calculating DC voltages and currents in arbitrary series and parallel circuit will all help you design robust power supplies.
That's just my opinion, and I can't argue that
@BobK has a valid opinion that is the exact opposite. After all, we both took the same route when starting out but have arrived with different opinions. As always, your mileage (or kilometers) may differ. Just remember the whole point is to have fun. Without the fun part you may end up just having a job.
Hop