Returning to the US from India as a US Citizen

I have several questions about returning to the US from India as it's my first time flying internationally alone. I booked tickets on United Airlines' website. I would travel with both Air India and United Airlines. My flight with Air India is within the country, and I have a layover at DEL. I would first travel from BLR to DEL via Air India and then DEL to the US via United.

I'm confused because United Airlines only sent me a receipt for my purchase. I'm not sure of how I would obtain my boarding pass.

Will they not let me in the airport without a boarding pass? I would board at BLR, and I was thinking about getting the boarding pass at the airport itself instead of printing it.
Would I go to the Air India counter or the United Airlines counter when I arrive at BLR? If I go to the Air India counter, would they be able to give me both of my boarding passes?
Would a US passport suffice for domestic travel in India?
When looking at the allowed luggage dimensions and weights, should I look at the ones for Air India or United Airlines?
Is there a way I can find out the gate I will arrive at in advance?
Is it okay if my carry-on bag does not close? I.e. it has a clasp, but it does not have a zipper and the inside contents are visible
Will my carry-on luggage need tags or should I add an identifying sticker?
How can I access my boarding pass to print it out?

I apologize for all of the dumb questions. Thanks!

usa india

Best Answer

Adding just some details:
In India, some (all?) airports have outside security that only let you enter if you have proof of being a passenger. This can a paper boarding pass, but it can also be an electronic boarding pass, an electronic ticket, or the simple confirmation email from the airline (yes, that would be easy to fake, but I guess the people they are worried about don't have the means to create such a fake).

Luggage: the operating airline of your first leg defines the size and weight limits. So don't look at United, but whoever operates the first hop you fly.

Gates: For international flights, both departure and arrival gates are typically published only on short notice, for security reasons. But why would you care? You'll have plenty of time to find out once inside the airport, and nobody can meet you at the arrival gate anyway.