Firebase and authorisation in the app

Hello.
I want to know how to make the best way an authorisation in apps and which materials do I need to know how to work with the firebase?

And also I wanted to ask what would be better, to make many simple apps or to make 1-3 very high-quality apps, to make more money?

Because I read from some site that for monetisation I need to learn a firebase and to make an authorisation in the app. But now I understand that sth was changed and I cannot do the same things like inn videos. Therefore I asked this question about firebase. Also I have read some nice stories about how people make money from apps without any communication connections and without much money(like 500$, I know that it’s not much money but now I have no even this from my app).

What do you mean by this? A way to have a login for the app?

The best strategy for an app to make money is to solve a problem!! Which is a business like ideal but it’s true. Your app can be great and have tons of features, but if you don’t solve a problem for your users you won’t make any money or have any users

Even if a app solves some problems it doesn’t mean that it brings some good incomes. It also depends on your marketing of the app, on your some tricks like the authorisation(because user in this case knows that he need to use this app before it closes), etc. For instance, I know some apps that don’t solve any problem but the average incomes are 240$ pro month (apps like notes but with another design). Also, I believe that there is no more than 20 apps that can be installed that can solve any problem in your life that soluble via apps. Others do nothing important. For example, how many people use apps from appstore or play market to learn some languages and how many of them become to know the language? You can not to answer, it’s rithorical question. Therefore I want to ask you what do you mean by “solve a problem” if all problems are solved by other apps. And why do you write me that app brings money only and only if it solves the problem(if I correctly understood you)?

If your goal in life is to make money from an App then you might be very disappointed.

The people who have made money from Apps have created an App that solved a problem that THEY had. It just so happens that there were a lot of other people out there who have found that App very useful and were prepared to pay for it.

So you should look at it from the point of view of your own life experience and what you do that could be made easier with some kind of App.

It may not necessarily need the user to Log in if the data it stores can be stored locally. A multi-user App that requires Firebase is eventually going to cost you money if the volume of data stored in Firebase increases beyond a “free” account.

I don’t understand which problems can be solved by apps that single developer can make except reading books in djvu, pdf or another format, watching videos, communication problems(which were solved by many apps as VK, Telegram, WhatsApp etc), and solved by the built in apps problems. Possibly problems with the sport but there are too many apps for the solution. Can you give some nontrivial examples of the apps that solve some problems but not very recognizable(the apps that knows almost anyone)?