I wrote a tool that downloads firmware from official Apple repository.
I know there already is an un-official website for this, but I had the idea as how to do it in SwiftUI.
Besides, the app is able to provide more info, and check the SHA1 hash for file integrity. Via Virtualization.framework
it can also auto-detect the firmware for you Mac.
You can check it out on GitHub.
PS Hope this is the right place for this. Feel free to check out the project and contribute if you want to.