Unity and Apple both have some bad habits which sometimes don't play well together. So, things to check (take notes on the results):
1. Try the file from itch to see if Steam's the problem. (It's probably best to completely close out of Steam for this.) I'm assuming other Steam games work as expected.
2. Verify the file integrity: Load Steam. Right-click the program and select "Properties…" then "Installed Files" followed by "Verify the game's files are installed correctly."
3. If the UI demonstrates that it's attempting to respond no matter where you click, it's not the resolution scaling. Try to activate different controls (in different parts of the screen) first on loading (before it grinds to a halt) to see if it's responding in the right spot and if it's specific items that are the problem.
3b. Wait it out. Go make a sandwich or something and see if the problem resolves itself with time. (If so, reload and see if the problem stays solved.)
4. Drag the window out of the way and see if the OS popped up an ask for some kind of permission (like disk access or something). You can try this before and after attempting to work the program. These usually appear in the middle of the main screen, but not always.
5. Load Activity Monitor and look it over while sorting it by memory and CPU usage. See if it's using up more than expected and if the amount changes when you try to use the UI. If the CPU or memory usage becomes unexpectedly high or low it's a sign that something's bugged.
6. When you close the program, check Activity Monitor to make sure it's actually closing. If it leaves a process running, even at 0MB/0% of CPU, then it's not closing clean and can affect future attempts to run it.
7. Go through the Security and Accessibility sections of System Preferences and see if the program has an entry in there. It's possible that it asked for access and thought it was rejected.
8. If none of that solved it, and you're sufficiently brave you can update to the newest version of macOS and repeat the steps to see if anything's different.
Hopefully this will give you the shape of the problem so it can be addressed. If you have access to another Mac you might want to test it there (use the itch version b/c Steam). The dev may need to recompile if there have been any Unity updates, but let's try to get it working before going there.
[I'm not saying it runs on my Mac; I literally just learned about the program and saw the ask for help, so I'm going by my professional experience and education.]