If you're using Git on your Mac to merge branches, add the following to the end of your local .git/config:
[merge]
tool = unityyamlmerge
[mergetool "unityyamlmerge"]
trustExitCode = false
cmd = '/Applications/Unity/Unity.app/Contents/Tools/UnityYAMLMerge' merge -p "$BASE" "$REMOTE" "$LOCAL" "$MERGED"
To use the tool, do
git merge [branch-name]
If there's any conflict, do
git mergetool
Hopefully UnityYAMLMerge will help you resolve the conflicts in .unity files.
If you're not using Git on a Mac, please refer to https://docs.unity3d.com/Manual/SmartMerge.html.