They've had an "app" since the early 90s. It's called a "Factory Service Manual" or "Alldata"

There's no way an inexpensive phone app could be made comprehensive enough to be a nothing-to-something tutorial, let alone on the fly. Something along the lines of "Siri, how do I fix <problem>," would set people for getting in way over their head.

Haynes has an iPad app. I work on enough cars that aren't mine that (if I had an iPad) I'd consider it (but usually I can find a paper manual for under $10 used).

In the same way that software is built on thousands of layers of abstraction there's thousands of small steps between only knowing how to drive and designing your own parts for an engine build. 90% of it can also be learned for free on the Internet, just like coding.

Pretty much any service or repair is a collection of small simple tasks. Think about the competency level of some of the people that work on cars...doesn't seem so hard to do now does it?

Get a Haynes manual for your car and read it cover to cover. Even if you don't do your own work, being familiar with the systems in your car will help prevent shops from taking advantage of you.

Working on cars can be like meth. Once you start you're hooked. It often goes something vaguely like this:

If you can add gas you can add windshield wash...
Checking your tire pressure isn't a big step from there...
Changing your oil is easy to do...
Changing a belt isn't really and harder than changing oil..
Changing spark plug wires takes literally no tools...

You do more minor preventative maintenance and replacement of wear parts, become more competent and acquire a few more tools.

Eventually it starts being practically impossible to justify paying $60-$120 for shop labor and 200%-%400 parts cost when I know how easy and cheap some stuff is...

"Doing a brake job myself will me cost 1/4 or less of what it would cost to have shop do it and comes with the piece of mind that I didn't cut corners..."

"How can I justify paying someone to install a new bumper cover when it just needs to be snapped on and screwed down...and the shop that replaced the one on your parents car couldn't be bothered to use the screws..."

"Rear wheel bearings cost under $20 for the set, a torque wrench costs $10 and I don't even need to remove the tire to change the bearings. Hell yeah! This is refreshingly easy..."(90s Tercel, if anyone was wondering)

Fast forward a three years and one swapped engine:

A sensor fails in a particularly odd way, you diagnose it correctly because you know how the system it's a part of in great detail, that's the value of experience...

You're stranded by a minor problem while riding in someone else's car on a Saturday you convince them to have it towed to a shop or Autozone. You've never done that particular repair on that particular model but Autozone's website says it's in stock at that location and they're open until 10...$40 and 1hr later you're on your way to Walmart to buy a clean shirt...

You get a call from your dad "Hey, I just got out of my dentist appointment. Remember that joint on the exhaust that you said was pretty bad?" and reply "Is there still <repair stuff> in the trunk to the front left of spare tire or do I need to bring it?"

IMO, just start learning about and working on your car. If you want to know something Google it.