Hi Michel,
If you don’t mind me asking, do you sell more on IOS or Android? If you had to choose only one platform, which one would it be? (sometimes, even if we sell a little more on a platform, we are more comfortable on another one for many reasons, notably a preference of development tool, etc.)
The overall iOS market has higher sales volume than Android.
Just to show that there is no silver bullet or one-size-fits-all kind of advice, I make a living off my app and I do not participate in any app stores and do not advertise at all. If you can find a niche and corner it, word of mouth will do the rest. But this is absolutely the exception, not the norm.
I sell more on iOS.
Check the 5 power disqualifiers for your market/app: https://www.perrymarshall.com/44067/5-power-disqualifiers/ .
Selling to broke students for instant is not a good idea. Selling to their affluent parents will work much better.
Never lose perspective that unless you are dealing with the client for bespoke development, all sales of ready made software are self service.
You need to put up an attractive offer, and make sure it is visible by the public you target. If you have a web site, for instance, it is paramount that you make sure it is visible in Google.
This can help: SEO Starter Guide: The Basics | Google Search Central | Documentation | Google Developers
It will show you how to build your pages so they index well in Google, and potentially rank toward the top.
Also, you must make sure Google knows about your site. This is where you enter your site URL and sitemap: https://www.google.com/webmasters/tools/home?hl=en
The nice thing with app stores is their huge visibility in search engines. Also, since they are in the dock or main screen for Apple, in Windows, and Android Play Store, users go directly there and never bother searching on Internet. In practice, stores benefit from a huge captive customership.
Just a small contribution to the debate:
I do not sell a lot at all 
- but same applications are sold better for iOS than for Android (for Android I earn about 30,- USD a month alltogether ).
Start learning xojo web and document yourself in the saas ecosystem. after, get a brilliant ideea of a software that solves a “pain” for the customer, easy and fast. Hint, get into the building api’s for different needs and list them on api marketplaces.
What type of api marketplaces @Gabriel_L ?
You need to come up with an app that people will pay for. In my day job (engineering), there are quite a few specialised niche applications that customers pay $4k - $20k per-seat for, an in some cases annual “maintenance fees” around 10% of the initial cost.
While corporates are invariably Windows environments, “working from home” is changing that as some are starting to allow BYO devices to connect remotely, and many have Macs.
You need to find industry forums these types attend, and be prepared to run a presention or workshop where you can showcase your app and hope they ask “what was that app ?”, or simply attend as a vendor. And be ready with marketing material - website, flyers, white-papers and so-on.
But that’s only the start - the big money is in “consulting services” training users and getting new customers projects up and running fast - at serious $ per hour.
Then there’s the prospect of training and licensing specialist consultancies to provide those services rather than you having to do it all or build a company yourself; this can generate a modest licensing or royalty stream.
somehow a niche product will generate money in apps stores.
I can attest to that. I have been getting a steady income since I started back in 2014 with the MAS. Now I got the same apps in iOS, Android and Windows.
The thing is, to avoid doing “me too” apps, and to put enough work into the apps for them to be truly better than rivals. In app stores as in any commerce, instant success may perhaps exist, but much more frequently steady and consistent efforts lead to the best results.
I may be wrong, but I also believe anybody who is there only to make money will probably face severe disappointment.
I’ve been selling Mac apps since 2007. never got rich. i did do pretty well for a while, but not so much these days due to changes in my niche industry. I never got into the Mac App Store till about 5 years ago I think. It was a major pain for me, bc I had to practically rewrite my whole app due to Apple’s lack of support for 32-bit apps and what they call legacy software (QuickTime). At first all I did was make a website and list my app on MacUpdate and download.cnet.com (or whatever it used to be called). That was free to list them there. Mac App Store charges you $105 per year to be in the store and if you make less than $1 million in sales they take a 15% cut. 30% if you’re over a million in sales. Getting my apps approved by Apple was a major pain, but it’s not as bad now it seems. They do have a gauntlet of signing certificates and such, that really is a pain to deal with. Thanks to AppWrapper for simplifying that to a large degree. I make more off of my sales outside the MAS still for some reason, but as long as I at least cover my cost to be in MAS, I’ll probably continue there.