Oh? I though I had to buy a $99 developer account,and connect the device with my account to be able to build it and install it. I guess I missed something important in the docs.

Anything that requires access to Apple's service – TestFlight, push notifications, Game Center, etc will require payment. If all you want is to stick an app on your phone that's free (as of ~5-6 years ago?).

There are limitations. The signature on the binary is only valid for 7 days I believe, so naively you'd have to rebuild/reload every week, however there are certainly automations that do this for third party apps with a little daemon running on your Mac in the background, and I suspect there are equivalent automations for your own apps/open source codebases to reduce the hassle.

So "free" but still a lot of hoops to jump through. For android I just plop down an apk (like a normal OS).

I worked on my first app for a year before getting a developer license and uploading it to the app store. I had no trouble installing it on my device. You missed something.

You can only install three such apps, they expire every seven days, and they cannot use important features such as push notifications (or include Network Extensions, which notably means you can't write an obfuscating VPN app to help people bypass firewalls in countries such as China).

There are many entitlements that the free account can’t have. There are also other provisioning profile management bugs with free accounts that apple hasn’t fixed.

No, it used to be like that but not anymore. If you want your own app on your own phone to last more than 7 days you need to buy the $99 membership.