Would j20 be used at china -india border?
It's not at the moment because the Indians have totally backed off and surrendered parts of two disputed regions. If however the Indians come back for a fight, then it's uncertain. IAF doesn't have Rafales ready for fighting. It takes many years for pilots and ground crew and the commanding structure to get trained on new fighter and then develop tactics after all this. Plus they've only got 2 or 3 delivered into IAF so far I think.
The most capable fighters in IAF are the Su-30MKI and Mirage-2000. The Mig-29s are very old and the French fighters carry better weapons than even the MKI. Most of the western and newer Indian missiles are not yet integrated onto the MKI and even then the Astra has pretty poor range, no dual stage motor/s, no dual seekers, and India doesn't have much experience at all developing high end missile software and hardware components. Astra being their first attempt. Meanwhile Chinese ones have been learning from western missile tech and leading ones too since the Sino-Soviet split and built upon those lessons many generations already. PL-12s will suffice, no need for even PL-15s and PL-21, PL-xx unless they just want to fire some for live testing.
J-20
should be used in case of IAF vs PLAAF. It'll be a once in a lifetime opportunity to verify training, tactics, and lessons learned can be applied to modify them and implement in further training with PAF in future. If JF-17 and F-16 could absolutely devastate IAF, Indian air defences, blind and subdue communications and radars, then what would J-10C + J-16 + J-20 + hundreds of UAVs + dedicated early warning, AWACS, tankers, dedicated electronic warfare, and superior air defence achieve? Even if PLARF spares Indian airfields from ballistic missile attacks and PLA spares them from receiving hundreds of CJ-10. Could explain why the Indians totally gave up on their claims in Galwan and Pangong Tso in the span of a few weeks and officially announced it lol all without firing even a bullet.