Is mobile app the only way to create side projects (for good user count)?

I will say normally users prefer web to test things out because its easier. The friction of having to download an app is really really really high as opposed to clicking the link and being able to reach your target action in a few clicks.

Also, no one wants to download yet another app which can just be a website and then get tons of notifications, it then forces you to sign up and clogs your inbox.

Just need to make sure your app is actually designed well for mobile users. Most web apps are horrendous for for web use bc its treated as second class citizen.

In fact when designing web apps you should almost always design mobile first bc when you tell someone to check out the app they will most likely check it from a mobile phone even if they dont use it on mobile

Because the mobile app is the only way to get users

That's not necessarily true. A user is a vague term and can mean anything. I prefer to count a user as someone who does a core action on your app/meaningfully interacts/contributes to your north star metric. For example on taro it might mean upvoting a post or replying or spending > 30 seconds watching a course

Is mobile app the only way to create side projects (for good user count)?

Definitely not:

• In the course itself, I show the web version of Random Name Picker which is even more successful than my Android version! That name picking website gets 10 million+ visits per months.
• There are tons of non-mobile GitHub repos with thousands of GitHub stars
• There are tons of ML models on Hugging Face with thousands of likes

It's just that I've found mobile to be the easiest to get users and claim credit for them due to the more enclosed and less competitive ecosystem alongside the social proof being baked into the app listing. This is a pattern I've seen across my mentees who I have coached to making side projects with 10k+ users - However, you also need to take this with a grain of salt as I will obviously be the best at mentoring other mobile engineers given my expertise.

I have one more question: Is it a good idea to learn mobile app development (for Backend or Web FE dev) if you have an idea that leads you good user count?

Probably not. If you already know front-end web, just build it on web.

The only time it really makes sense to pivot to mobile for side projects is if you're very early-in-career, like an intern or newgrad who doesn't have much existing expertise in anything.