I do all processing using 3rd party software. The few times I fiddle with he GoPro app I found it nothing more than a frustration.
I am curious as to whether one can process complete videos using the Mimo app alone. I have a young friend that is contemplating the Osmo Action. However, he has no computer. Everything will need to be processed on his phone. Is this realistic?

He's not planning to be "cinematic". He simply wants to share his videos with friends.

What can be done with the Mimo app?

Anyone ... ?

There are lots of apps for video editing on mobile phones. I use a Mavic Air and so far I've done all my video editing on my iPad and iPhone XS Max since I don't own a computer.
LumaTouch is the best free app for video editing for beginners IMO. It has more features than iMovie and is not as complicated as DaVinci Resolve or others.

Thanks.

Unfortunately, he has a Huawei ...