Need older version of iMovie

I inherited an older iMac from my mom. I updated the operating system as far as I could go...which was macOS Monterey version 12.7.1. When I did that, my iMovie app got greyed out with a white slash through it. If I try to launch it i get the error message: "In order to use “iMovie.app”, you need to update to the latest version. The version of iMovie installed on this Mac is not compatible with macOS Monterey. Download the latest version from the App Store." The App store will not let me download the most recent version of iMovie because my OS is too old. I tried the suggestion about moving my icon to the trash (but not emptying it) etc. and that did not work. Do I have any other options? Why wouldn't Apple have a site I could go to download an older version of iMovie? Is there a "reputable" site where I could download this? In addition, I suspect the same thing is happening with my Garage Band app. Halp!