My daughter's iPhone xr says it is sim locked and it is one that her friend gave her about a year ago and she no longer goes to school with her friend nor talks to her so we don't know what carrier was originally to get phone unlocked. My question is how do I find out original cell carrier?
Yet another possibility is to get the IMEI→iPhone: Locating the serial number, IMEI, ICCID or MEID and call each of the 3 US carriers, and ask each if it is their phone and can they unlock it.
Can you get into the phone Settings?
If so, Settings > General > About
Then scroll down to the "EISM" category.
Look at "Network" and "Carrier"
Alternately.....
Tap Settings
Tap Cellular
Look just above Wi-Fi Calling
The carrier's name will usually appear here AT&T, Verizon, etc.
Hopefully, your daughter's friend followed the instructions in this Apple Support document, so you can avoid an Activation Lock on the phone, another type of security measure that is not related to Carrier Lock.
What to do before you sell, give away, or trade in your iPhone ...Official Apple Support › en-us
Finding original cellular carrier
