Its always first class jobs.
If you do it.Do it right, First time every time
Because if you cant do a better job than the manufacturer, why bother? They have the advantage of proper workshops, teams of experts, outside suppliers/machine shops that make complex parts to order, and a production line. And have the luxury of a development time/budget to problem solve. Mine has to be better because I get just one go. Must look, and work right from the start. Or it wont end well... A few tiny errors or problems will end up with a chair thats not as good as a bought one. And you will wonder why you bothered and regret ever trying. And you dont see the parts I screwed up, drilled fracionally wrong, or that ended up needing a redesign.
And is just practise and a grasp of whats available / possible and of materials. I made and saw a lot of stuff when building/racing drag bikes, nitrous systems, rolling roads, in my own businesses, fitting kitchens, bathrooms for my dads business, and in various hobbies.
And hobby stuff like building model planes, steam engines, miniature gas turbines/pulse jets etc. Its a lifetime of messing about with mechanical/electrical stuff. It lets you see how stuff goes together. And think of similar solutions with the least work, best solution.
But now as you say, I know how, have the knowledge but need 3 helpers to drill a hole!
And this
download/file.php?id=3955&mode=viewfor eg is an overweight bodge to fit a wheel. Its strong and it works. But a single manufactured hub purpose built from 1 part would be better. But I dont have the required machine shop... So its a solution but not the best solution.