Open source. That means that they'd have to either prevent the downloading of the source, prevent it compiling, or prevent running something that you compiled on your own box. Any of those three seems to be guaranteeing that Europe will not have any leading role in computers for the foreseeable future. Any of those three also seems almost impossible to enforce.

There's no way to build apps locally on most smartphones which are the majority of end user devices.

Open source practically died with personal computers.

When did personal computers die? Most desktops and laptops are fairly open and although android phones are not as open you can build software on your laptop/desktop for usage on your phone.

You could also use something like pinephone or librem. You wont have access to a lot of android tech but the most important functionality. A web browser, sending sms, email, making calls all work.

I agree. Currently not many people use open source on their phones, but that could change, and things like F-Droid are ready to be used.

It wasn't that long ago that encryption was banned in France.

Source?

I can envision all sorts of workarounds to this constitutional issue. And, Congress does this sort of thing all the time.