It gets easier to envision if you think less of "apps" and more like "systems which will exist within an existing business which already has many, many millions

It gets easier to envision if you think less of "apps" and more like "systems which will exist within an existing business which already has many, many millions of dollars flowing through it." If the new/improved system significantly increases how many millions flow through the business, then that is the only thing that needs to be said, right? After that it's just administrivia what sort of charging model gets worked out. If you're an insurance company and you need a computer system to support adding a new line of business to your products available, there are very specialized programmers who can help you do that. (Or you can hire Accenture and, for a 150% premium, you won't need to know that any programmers were harmed in the making of your system.) There are virtually infinite examples of this.