Unlike 4G, in 5G users never send their permanent identities over the air.
In the case of 5G, IMSI is transformed into a new Identity named SUPI (Subscriber Permanent Identifier) which is never shared on the air.
Instead of sending SUPI over the air, UE converts into a Privacy-preserving concealed ID named SUCI (Subscriber Concealed Identifier) which is a one-time use identity.
Then SUCI is sent to the home network and in the home network SIDF (Subscriber Identity De-concealing Function) which is a sub-function of UDM coverts SUCI into SUPI which is further used for fetching Authentication data from UDR or ARPF.
Also aligned with the 5G mission of ZTA (Zero Trust Architecture), SUPI is shared with the serving network only after the completion of the Authentication procedure.