Not all of it, of course. But many common repairs and maintenance tasks can be handled by a reasonably attentive amateur with the right tools.
For example: the replacement of a capacitor. A local AC contracting company quoted me a price of $384 for this repair. The necessary replacement part costs $19.95 on Amazon. You’ll also need a multimeter, which can be purchased on Amazon for a very moderate price.
In case you haven’t noticed, I’m a writer, not a handyman or an engineer. Bookish types like me typically don’t like to get our hands dirty. Well, I discovered a few years ago what a self-defeating attitude that is.
I solicited a quote for a necessary home repair, and found that the company in question wanted about 10x the reasonable price for the job. Two other contractors in the area wanted 2 to 3 times the reasonable price.
I define reasonable here as: parts + labor + (reasonable) overhead + (reasonable) profit.
That said, there are limits to what you can do for yourself. The installation of a new AC system is beyond the scope of any homeowner; and environmental laws require that a licensed professional complete the work.
But you should educate yourself about what is actually involved in common home repairs. And never accept a quote from a vendor that isn’t broken out as described above. Most contractors who serve homeowners don’t want to do this, because they don’t want you to know how much they’re overcharging you.