It used to be Mac-only, because the work is done by driving a real browser on a real desktop and that part was written for one operating system. It is written for both now.
It needs macOS 14 or later. Everything lives in a folder you choose, so a Mac that isn't your daily machine still works.
There's a free iPhone app, and it won't help here: it has no books of its own and shows nothing until a Mac has closed something.
Windows 10 or later, 64-bit. The app is free and runs the full monthly close; it does not read a photo library, so photographed receipts are found on a Mac only.
Nothing to sign up for. If you want to hear about releases anyway, your account page has the switch.