I'm only aware of this in generalities. My understanding is that platform providers can be lawfully compelled to read telegraphs or open envelopes, if it is technically possible for them to do so.

A "true" end-to-end encryption scheme would mean that the post office physically cannot open the envelope. In practice, most of the time they could but choose not to, and this is the type of system which can be overcome by a warrant. This happens because a) e2e encryption is bolted on-top of a non-e2e system b) a "true" e2e system like that requires the sender and recipient to manage keys, which is a hassle so usually the platform does it for you c) platforms get political brownie points for being friendly with law enforcement.

Standard messaging platforms resemble an old-school telegraph. End-to-end encryption more closely resembles physical mail.

To send a telegraph, you beep out your message. This gets sent to a telegraph station, where a person listening writes down your message, and then beeps it on to the next station down the line, where there's another person waiting. Eventually, someone writes down the message and hands it over to the recipient.

The important thing to note is that the telegraph operator reads the message at every hop. So if your telegraph operator knows your cousin, they'll gossip. And if the Government thinks you're up to Crimes, they'll watch over the shoulder of the telegraph operator to see if anything Looks Like Evidence.

Mail, on the other hand, is sealed in an envelope. And that envelope gets handed to a postman, tossed around by baggage handlers when it's put on a plane, carried around by another postman, and then delivered unopened to the recipient. No one else has seen the content of the envelope until the recipient opens it. It is a crime for anyone else to open this mail, even the Government if they're not going through proper channels (warrantee void where it's voided).

The encryption is, more-or-less, the envelope that stops non-recipients from reading. The "end-to-end" part is the fact that it stays unopened from the beginning of the journey to the conclusion.

This, of course, relies on trusting the postal system actually does what they say they do. The post office has actual laws that guarantee it works this way, whereas some service that claims end-to-end encryption does not.