There are a couple of questions on this forum that address international transfers at Delhi airport, and whether checked luggage has to be reclaimed in the process or not. This answer links to multiple reports of transfer desk staff taking care of the checked baggage, regardless of whether the two airlines have an interline agreement or not.
Is there a similar service if you are connecting from an international to a domestic flight or vice versa? As far as I know, all international passengers transferring to domestic have to go through customs at DEL (with all of their luggage). Is there some sort of "bag drop" counter after customs, like at US airports, which allows you to avoid going through the whole check-in process again for your connecting flight? If yes, when can you use it (e. g. do the two flights have to be on one booking, do you already have to have your boarding pass for the connecting flight etc.)?
On the other hand, if you are connecting from domestic to international, in my experience the baggage gets checked through to your final destination if all flights are on one booking. What happens if you have two separate bookings? Is there a similar "transfer desk" to those mentioned in reports like this one where people collect your luggage and re-check it for you while you just go through security and exit immigration?