Is it possible save data in a csv online for example on Dropbox or Google drive? I have seen that we can retrieve data from them but can we change them?
If you want to suffer for a very long time, then try the Dropbox API. If you don’t want to do this, then use the components that are in Thunkable X.
I need to save a csv on a web site or Dropbox or Google drive.
Not on my phone.
What is the fastest way?
Use WebAPI component for send data to web site (server script).
is there anly link or tutorial to do that?
https://docs.thunkable.com/web-api
Creating server-side scripts does not apply to Thukable X. You should find it yourself, depending on what technologies the server supports and in what programming language you can program (php, python, perl, cgi, с++ or other)