Developing a Wordpress theme - where to start?

I’ve looked at existing themes that I’ve bought and have implemented.
Woooah… where does ALL that code come from???

I’m guessing there is a lot of usage of ready made frameworks to start from?

In another post someone mentioned: Well first of all, you need Theme Options for theme and ‘Redux Framework’.
I checked that out. £200 per year for agencies.
Do you guys subscribe and pay for this?

I’ve seen other things like elementor… how does that fit in?

Hoping someone can give a few pointers.