Can you recommend an External Hard Drive that is compatible with Mac Catalina 10.15 and Windows simultaneously? I like to use my hard drive on both my PC and Mac. Thank you!

Any external drive will work. You need to format it to a compatible format that works with both OSs. ExFat can be read/write with both Windows and Macs. Be aware that Time Machine won't work with a drive formatted ExFat.

any drive formatted ExFAT or a NAS solution

you can format ExFAT in Mac OS using disk utility or in Windows

https://docs.microsoft.com/en-us/windows-server/storage/disk-management/initialize-new-disks

Caution: While ExFAT works, it can be quirky setting up. Format on Mac, may not write on PC. But format on PC and it might finally write on both. Try both ways, one should work.