Upgrading Server 2012 R2 Foundation Edition to Server 2016 anything

Sorry but there is no in-place upgrade path period from 2012 foundation.

https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-foundation/jj679892(v=ws.11)

 "Upgrade of Windows Server 2012 Foundation is not supported."

Since you have two and assuming the hardware is supported for server 2016 (check here and with manufacturer to confirm)

https://www.windowsservercatalog.com/

You could move the fsmo roles to other server then rebuild them one at a time.

I'd check that the domain functional level is 2003 or greater. I'd use dcdiag / repadmin tools to verify health correcting all errors found before starting. Then stand up the new 2016, patch it fully, license it, join existing domain, add active directory domain services, promote it also making it a GC (recommended), transfer FSMO roles over (optional), transfer pdc emulator role (optional), use dcdiag / repadmin tools to again verify health, when all is good you can decommission / demote old one, and move to next one.

Also at some point after I'd recommend migrating sysvol replication from older FRS technology to DFSR
https://techcommunity.microsoft.com/t5/Storage-at-Microsoft/Streamlined-Migration-of-FRS-to-DFSR-SYSVOL/ba-p/425405