Why does my game's Mac build not work?

I create a Mac standalone build:

164034-mac-build.png

I upload my game via steam, then try to play it from my Mac:

This worked the last time I tried it (years ago, before macOS Catalina).

It looks like my game is 32-bits and so it cannot run on Catalina. Why is my game 32 bits? How can I create a Mac build from Unity that runs on macOS Catalina?

My Unity version is 2019.2.21f1 Personal.

Thanks!

So turned out this was a Steam issue not a Unity issue.

On steamworks under “Installation > General Installation” I needed to add a launch option for Mac, and then publish my changes.