how could i download an installer without actually installation (build a bootable usb)?

hello all:

i messed up my secondary macbook pro, wiped out everything. so i need a bootable usb with installer for a clean installation. but i can't download an installer from apple store since it always attempts to update my current macbook pro, even i disabled all of autoupdate functions. my current macbook pro runs catalina and i have no intention to upgrade to big sur at this moment.

i followed the link https://support.apple.com/en-us/HT201372" but without macos installer i can't go on. i just need to download an installer to build an bootable usb.

please help!

_dave