People don't buy phone apps, they pay for services. Meaning that if you make an app, you won't make money for that app unless it is service based. There are a few outliers to this.

Games are different than this as people will pay a set amount of money for a game to own, but they will also continue spending money on the game via microtransactions or 'packs'.

Most people don't care about their security or privacy - but that's all the more reason to make it clear that you respect their security and privacy.

Everything should be opt-in, not opt-out in terms of data collection, marketing, and advertisement.

It should be very clear that if someone is not paying for a service or tool, that the ads are what provides the revenue.

Enterprise software is weird as there are many regulations associated with it and they company has every choice to go with an open source alternative, of which one most likely exists. In other words - don't aim for enterprise use at first.

The majority of people use their mobile device (Phone) as their gadget, they aren't using a desktop or a laptop if they even have one. If you want to attract the attention and get users, you need to meet them where they are. This doesn't mean make a Phone app that is just a front-end that goes to a responsive web app. This means, make a full-featured phone app. They have more power than early 90s and Y2K computers, take advantage of it.

If you limit yourself in an area of marketing or user-space, create an ecosystem where multiple platforms and apps can coexist and depend on the progress of another.