Tutorial: How to set up a Nextcloud development environment on Mac or Ubuntu?

Hi,
I first apologize, because my isuue is only marginally related.

I have difficulties to setup a Nextloud development environment on Ubuntu for compiling the Nextcloud desktop.
Specifically, I find no documentation how to patch Qt 5.15.2 from Download Qt: Get Qt Online Installer.
I assume my compilation fails because of a missing patch.

How do I set up such an environment?

Thank you,
Markus