Ubuntu’s popularity often makes it the default choice for new Linux users. But there are tons of other Linux operating systems that deserve your attention. As such, I’ve highlighted some Ubuntu alternatives so you can choose based on your needs and requirements—because conformity is boring.
I mean, in theory you could also put flatpaks onto a usb stick and symlink the directories. But nobody really does that.
But really, I think this could be a cool GTK app.
You would copy selected apps to the stick and include a program, maybe even with a GUI, that can then symlink those apps to the system you are currently using.