How can I set up an old Epson Workforce 3640 on my mac?

As long as it is networked (wireless./wired) the Epson WF-3640 is supported by Apple's built-in AirPrint drivers for printing and scanning on Big Sur.

With the device networked and powered, visit System Preferences > Printers & Scanners panel. Either add [+] the device and select the AirPrint driver or if the WF-3640 appears in the Printer list, right-click on it and choose Reset printing system… This will allow you to re-add the device, where you select AirPrint. Once re-added, you should be able to print and scan using Apple's Image Capture tool. AirPrint enables you to be free from Epson driver dependency.

Epson implies that their current driver and software are compatible with Big Sur for this device, but that raises big questions about how drivers/software released in Oct 2020 could have been tested on later Big Sur beta and subsequently released versions of Big Sur since Nov 2020. They did release an updated Image Capture (ICA) driver in Jan 2021. I would choose AirPrint over Epson's current driver support position.