Please add/correct as appropriate. (Need postcount of 90 to be able to edit)
(Note that information change dynamically and hence best to double check)
Thanks
desi

PLEASE NOTE that CERTAIN DOMESTIC FLIGHTS OF Air India (#010-399 and 900-999) are ACTUALLY segments of INTERNATIONAL FLIGHT and hence for the following discussion should be considered INTL flights.


TRANSFER at BOM

Air India and Vistara operate all of their international and domestic flights out of the same NEW terminal (T2). There is no need to change terminal or take any bus/taxi.

Low-cost airlines operate out of legacy Santacruz domestic terminal (T1) and you need to continue to follow the usual practice of changing terminal.
(For those not familiar with Santacruz Airport - CSIA - while both T1 and T2 uses same airfield and runaways, they are quite apart landside and can be 25-45 min by taxi depending on the traffic)

If first time in BOM then it is advisable to book connection with AI or Vistara to avoid this transfer between terminals.

INTL-to-INTL connections (suspended as of Jan 2022 due to COVID):

If both legs are on the same airline or on airlines that have interlined agreement, then your bags will be tagged to your final destination. But verify this fact when you actually check your bags at your origination and reconfirm by inspecting the bag tags.

Otherwise you have to go through immigration, collect bags and check-in again. This may need appropriate Visa too. If you have to do this, provide for 3 to 4 hours layover to be safe.

INTL-to-DOM transfer

Practice is similar to what you may find at major airports in the world such as JFK, LHR, etc. You have to go through immigration, collect bags, go through customs and then TURN LEFT upon exit from customs towards transfer area. DO NOT GO STRAIGHT into Semi-controlled reception area.

It is best to have entire reservation on single PNR to avail of better baggage allowance. Otherwise expect to pay high tarrifs of excess baggage. Also in case of delayed incoming flight, you will be most likely accomodated on the domestic portion.

Note that certain domestic looking segments of AI are actually onward leg of continuing International flight (#010-399 and 900-999). When connecting to/from these flights, treat them as INTL segments.

DOM-to-INTL transfer

It is best to have entire reservation on single PNR to avail of better baggage allowance. Otherwise expect to pay high tariffs of excess baggage. Your bags will be most likely tagged to your final destination when it is same airline or when airlines have interline agreement. Again, best to double check and reconfirm by looking at the bag tags. Otherwise, you have to collect bags at BOM and check-in again. Please check with your airline for the current process. Posts on this thread that are more than couple of months old, should be in "Trust but Verify" category.

If you are travelling by Air India and your incoming flight to BOM has been classified as international flight (currently applicable to certain AI flights only) then you would have already completed immigration process at your origin and you will simply follow INTL-to-INTL connection at BOM.

Otherwise, you will go through immigration at BOM airport even when your bags are checked to your final destination directly.