Thank you for your fast answer!

How do you rebuild and what does this exactly do? Is there any documentation about this?

I have seen the button on another odoo.sh database, but now it does not appear on my database...

Screenshot: https://prnt.sc/-AN3_6AgVm8D

The Rebuild feature is only available on staging branches. The goal is to reassemble a copy of the production database on a test environment in one click (and a few minutes).
Please note that this only includes data. If your code is not up to date on your test branch, it will not be updated.
On the provided capture you don't have any staging branches. To create a test branch completely like your production base, you can do so on the Branches tab, under Staging. (Warning, each additional test branch has a cost)

Documentation available here : https://www.odoo.com/documentation/master/administration/odoo_sh/getting_started/builds.html