Thanks a lot for the anwsers.
I will create a local development setup with composer. The first deployment ist all files with sFTP and the database with phpmyadmin.
And the further deployments? Can I deploy new local content without deleting old content already on the site?
My plan ist to start with a simple page template for a standard page. Then other persones of our homepagecrew can add content. In the meantime I will create an new page type for reports (text plus photo gallery). But if I deploy the whole thing again, all content ist gone, or not?
I am not sure, what is stored in the database and what is stored in the filesystem and how to sync everything between the server and my local develompent.
Sorry for my poor english - I am native austrian.
Thanks for any help and support!