Problem:
I want to overwrite the login.md of the login-plugin
Thought it could be done by copying the login.md file from user/plugin/login/pages to user/pages but it seems like I’m wrong with that
Do I have to copy the login template files from the plugin to my theme (otherwise I can’t choose it in the Admin?) and add a page in Admin or is there a better way?
Depends on what you want to do. You can configure the route in the plugin configuration to match any page created by you (see GitHub - getgrav/grav-plugin-login: Grav Login Plugin). The login plugin will then automatically redirect to this page for any login operations. It need to be a login form of course, so basically the login template or create your own template with a login form.
I just want to change the content
# User Login
to
## User Login
I wondered why I can’t select the login template on a page in admin on default.
When I copy the login.html.twig into my theme, it’s selectable.
But anyway’s It seems like I’m overthinking that 
I will create a login template in my theme and change the content there, as you said.
Thank you! 
@rokun, Try copying /user/plugins/login/pages/login.md into subfolder /user/pages/login/
user/pages
├── 01.home
│ └── default.md
├── 02.typography
│ └── default.md
└── login
└── login.md
In Grav, a page is always the combination of a folder and *.md file.
Thats it!
I didn’t add the /login/ folder in my /user/page/ folder!
Thank you 