Splunk Universal Forwarder logs to %PROGRAMFILES%\SplunkUniversalForwarder\var\log\splunk\splunkd.log on Windows.
Paths are printed exactly as Splunk Universal Forwarder writes them. Environment variables like %LOCALAPPDATA% and $XDG_STATE_HOME are never expanded, so a path can be pasted straight into a shell. Where a path differs between installer types or shipping flavours, each one is racked as its own card below - why that happens.