Hi @mastupristi ,
Thanks for your interest in NXP MIMXRT series!
Finding where this file is located requires locating its corresponding Tag to do so. Here, it should have a Tag with the MCUX_2.15.000 prefix. GitHub - nxp-mcuxpresso/lwip at MCUX_2.15.000

I got the exact location by looking for this file after getting the gitrepo locally via the West tool, hope this helps:
$$:~/MCUX GitRepo/mcuxsdk/middleware/lwip/port$ git log --all --oneline enet_ethernetif_kinetis.c
0d2ec91a (tag: MCUX_2.15.000_RW610_RC1) NXP lwIP modifications for SDK version 2.15.000 (RW61x) RFP3
8f30ea5b (tag: MCUX_2.14.0_MCXNx4x) NXP lwIP modifications for SDK version 2.14.0 MCXN10 RFP
e44c34fe (HEAD, tag: MCUX_2.15.000, manifest-rev) NXP lwIP modifications for SDK version 2.15.000
d8c6848a (tag: MCUX_2.13.3_RW610) NXP lwIP modifications for SDK version 2.13.3 (RW61x) RFP2
f95eb7f8 (tag: MCUX_2.13.3_RW610_RFP2_RC2) NXP lwIP modifications for SDK version 2.13.3 (RW61x) RFP2
e0550ec8 (tag: MCUX_2.13.2_RW610_RFP1) NXP lwIP modifications for SDK version 2.13.2 (RW61x) RFP1
02458d68 port: enet: add support for ARMv8
4ef7c6fd (tag: MCUX_2.14.0) NXP lwIP modifications for SDK version 2.14.0
4192f6c2 (tag: MCUX_2.13.0) NXP lwIP modifications for SDK version 2.13.0
2c1abcd4 NXP lwIP modifications for SDK version 2.12.0
Best regards,
Gavin