Use of encrypting data in transit over a HTTPS connection

I find certain APIs (that provide sensitive information) using algorithms like ECDHE with X25519, on top of the already encrypted SSL connection. This compels us to use libraries like Tink (when we have to use it in Android Apps) that provides such algorithms, though the classes it provides us for using such algorithms is explicitly marked not to be used in production.

Is there any reason this could have use cases when transmitting sensitive information?

Is it possible to setup email servers to encrypt communication in transit?

I understand that generally email communication is insecure by default and would like to know if its possible to setup email servers so that the emails get encrypted when sent between hosts.

I want to know if there is actually a way to include personal information in emails and keep it secure in a Business to Business Scenario (B2B). This usually involves a scenario where and email is generally generated or typed that gives instructions on who to contact or follow up with. It is intended for a human to read and not intended to be processed by a machine

I’d also appreciate any relevant explanation particularly details of commonly known approaches so that i can investigate further. Even if its at the level of a pattern and common strengths and weaknesses.

20 hours Layovers at Heathrow and Dublin airports. Do I require transit visa for both?

I hold an Indian Passport.

I am traveling from Mumbai to Washington DC via the British Airways flight which has two layovers: 16 hours in London and 4 hours in Dublin. I possess an F-1 US student visa. I would be reaching London at 18:25 p.m. and would depart for Dublin at 6:30 a.m. the next day. Similarly, I would reach Dublin at 8:00 a.m. and depart for Washington DC at 12:00 p.m.

I would also need to change terminals at Heathrow from Terminal 5 to Terminal 2.

I wanted to know whether I would require a transit visa for both the places as I am not able to get proper information about the requirements from the official pages. Reading the pages only show that you can transit without a visa if you hold an itinerary to your destination and hold the visa for the US.

Please help me as I am confused and don’t want to get in trouble at international airports.