Sidebar in Deliver

It seems that "Some text Widget" and the "Lorem Ipsum ..." is hard coded in sidebar.html.twig
But altering it in there would compromise future update or theme migrations, am I right?
So, is there another way to overwrite this part, writing only in /user/pages?

Sounds like an inherited theme would provide that for you. You can learn more about that approach at https://learn.getgrav.org/themes/customization#theme-inheritance

That works perfect. Thank you, Paul.