How can I organize test logs in a tree grouped by added steps of the keyword test?

Ha, that's from my webinar!

What you're looking for is the CreateFolder, PopFolder and PushFolder methods of the log object:

https://support.smartbear.com/testcomplete/docs/reference/project-objects/test-log/log/createfolder.html

https://support.smartbear.com/testcomplete/docs/reference/project-objects/test-log/log/poplogfolder.html

https://support.smartbear.com/testcomplete/docs/reference/project-objects/test-log/log/pushlogfolder.html

I actually created wrapper functions for this that provides a default styling and a single place to update / expand based on needs.

function indentLog(sDescription) {
  let oAttr = Log.CreateNewAttributes();
  oAttr.Bold = true;
  let step = Log.CreateFolder(sDescription, "", pmNormal, oAttr);
  Log.PushLogFolder(step);
}

function outdentLog(sExpected) {
  sExpected = sExpected ? sExpected : "Finished";
  Log.Message(sExpected);
  Log.PopLogFolder();
}