I have a cheap flight booked to Bangkok that connects in Mumbai where both legs are seperate bookings. According to the website I booked it through (kiwi) I will leave the transit zone in Mumbai and need a transit visa but through my research it seems you only leave the zone if you recheck luggage or go to a hotel or something like that. I will only have hand luggage and my stop is only a couple of hours so no hotel required. Will I still need to go through immigration?
Assume you will not be let onto the first flight unless you have the right to enter India, as that is the standard for most airlines.
Whether you need to go through passport control, customs (even without hold luggage you may be checked) and check-in and security depends on the actual airport and in some cases the combination of flights.
I am not familiar with the airports in India, but it is quite common world wide to send all passengers on incoming flights through immigration, and have them re-enter the airside of the airport after checking in for the next flight.
You might be lucky and stay airside, but you will still need to be allowed into India, as the airline can not be sure you will be allowed on your next flight and they are responsible for you entering or leaving India.