What's HLR? Can you explain?

The Home Location Register (HLR) is the telephone-provider-specific database that routes calls coming from other providers; while originally a 2G term (it's HSS in LTE and UDM in 5G), people still just call it a HLR lookup. The actual HLR lookup itself goes to a separate country-wide database that maps phone numbers to the cellular or SIP provider. So, when you place a call, your provider can use that database to then lookup the HLR database and reach out to it to further route your call to that person's specific location.