Different methods is suitable for different situations. I do code review in 3 ways: write a comment on the PR, send a directly slack message, and pull down the branch to refactor and go over it with the reviewee.
Write a comment on the PR. This method is suitable for one-off comment or suggestion, you can use it to show your ideas, to point out questionable code, to report a defect, to show code examples.
Send a slack message: This method is suitable for situation when you are not sure what the code is doing, so you can ask questions on slack.
Pull down the branch and refactor code yourself: This method is suitable for the code that requires extensible modification that cannot be easily expressed on a one-off comment. So you can do the refactor yourself and go over the changes of the person who wrote the code.
