I need to apply changes introduced in one branch to another branch.
I can use cherry pick to do that. However, in my case I want to apply changes which are relevant only for one file, I don’t need to cherry pick whole commit.
How to do that?

You have different options based on what you want to achieve:

If you want the contents of the file to be the same as on the target branch, you can use git checkout <branch> -- <filename>. This will however not “cherry-pick” the changes that happened in a single commit, but just take the resulting state of said file. So if you added a line in a commit, but previous commits changed more, and you only want to add that line without those other changes, then a checkout is not what you want.

Otherwise if you want to apply the patch introduced in a commit to only a single file, you have multiple options. You could run git cherry-pick -n, i.e. without committing it, edit the commit (for example reset all files using git reset -- . and only add the file you actually want to change using git add <filename>). Or you could create the diff for the file and apply the diff then: