Yes, you need to setup public keys. It is also documented (well, mentioned) on the wiki.
git clone -b slfo-1.2 gitea@src.opensuse.org:pool/qt6-base.git
It is the basic GIT, nothing openSUSE specific. But you do need to know that Leap 16.0 sources are in the branch slfo-1.2.
Historically Open Build Service had two components - the build engine and the Source Code Management system. The package sources that build engine on the build.opensuse.org were using were kept on the build.opensuse.org and associated with the build project.
It was decided to migrate SCM to GIT. But to build using sources from GIT you still need the build project on build.opensuse.org. You can create this project manually and import sources, but then you also need to replicate project config to match Leap 16.0 manually. For this reason it is always better to fork existing project (I hope it will inherit the proper config like with osc branch).
Then why are you following the instructions for the maintainer?
### As a maintainer, you may want to work directly in your repository
And it quite clearly says later
### If you are not the maintainer, you will not have write permission later on. So you may want
### to fork instead the repository and setup a build project.
“Looked” is not enough. You need to actually read it and understand. It is not suitable for blind copy-paste.
Make a step at a time. Use the sources from the KDE:Qt6 project you already built successfully. Implement your changes there. When you succeed, you can then return to the “true” Leap sources. Maybe something will improve until then (documentation, workflows) to make it easier.
You hit it in the middle of OBS to GIT transition. There is no established workflow, there is no good documentation. The existing wiki articles assume previous experience with traditional OBS. Even seasoned packagers have problems with the current state. For this reason ignore it for the moment - focus on solving your problem with Qt and try to integrate the fix later.
And this topic really belongs to Open Build Service, not to Open Chat.