Here's a quick video, installing Big Business Client 13 on new iMac.
MacOS Catalina and Big Sur protect you from malicious code by asking for all Installers to be Notarized as only containing 64-bit code. Big Business includes 32-bit code to connect Old Client, support 32-bit Micosoft Office, and more.
To install on macOS Catalina or macOS Big Sur:
1. Run the installer and click OK for not scanned.
2. Go to Apple menu, System Preferences, Security.
3. Click Open Anyway, then run the installer again.