I would also recommend https://www.atlassian.com/git/, it has a lot of helpful tutorials and articles.
Git subtree will not change the code automatically, it creates a copy of the subtree so you can make changes to it and then push it back to the subtree if you want to share your changes.
The closest to automatically update the code between two solutions would be symlinks. Check out https://github.com/git-for-windows/git/wiki/Symbolic-Links#creating-symbolic-links. You would still have to push/pull manually to make sure your repositories are up-to-date.
You can merge subtrees from within Sourcetree too, it does not have to be done from the command line. It has been a while since I used it, but I did notice that when you upgrade Sourcetree it used to loose the connections to the subtree repositories so you would have to manually add them back, not sure if this has been fixed yet.
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