How do I bathroom / kitchen / extension? etc.

I'd be interested in this.

I always assumed that it's mainly going to be interior stuff.

Tbh nothing.
The big things are the planning, the steel and the foundations.
Things like kitchen are just garnish.

If you have the time to commit to it - with your skillz- you could defo design it in a way that you DIY elements. If you accepted timber frame construction and some kind of cladding externally you avoid messy specialist stuff like brickwork. But I'd reckon you'd need a careful plan from the outset on who does what when and it would be a massive commitment time / stress wise. Doing any decorating (paint) is an easy save of a few grand - but again time...