i am pretty new about all this kind of technologies but i want to put some documents (.odt or something like that) online and want to edit these documents from different devices (pc, phone, etc) without having to download/upload all the time.
What kinds of (privacy friendly) programs/apps are used for this?
Only office. Afaik, apple also opened its web version of office apps. I don’t have any proof that they are privacy friendly, but I believe it cannot be worse than Google and Microsoft.
If you have a Nextcloud instance in a VPS or local NAS with online access, you can install Collabora: the online LibreOffice. If docx compatibility is important, IIRC you can also self host OnlyOffice this way.