As compared to reality, where Telegram openly tells you the opposite
I don't agree on this one. Telegram claims to be an "encrypted messaging app", aping the claims of apps like Signal and WhatsApp that employ E2EE by default. To the average non-technical user who doesn't then specifically read an FAQ page, it sounds like Telegram employs E2EE by default. Only when one reads help documentation does it disclaim that most chats aren't protected by E2EE.
It's the governments that are complaining about Telegram's encryption. While being "mysteriously" quiet about Whatsapp.
So before we get to WhatsApp, let's start with Signal. Signal does not log IP addresses, and designs everything including profile data (profile name, profile pic) and message/group metadata (group name, group participants, sender of a message, etc.) E2EE. When served a subpoena, all Signal can disclose for a given profile (phone number) is the time that number first registered for Signal, and the time that it last connected to Signal. They can't disclose any message content because the design of the service does not give them access to it.
WhatsApp E2EE encrypts the contents of the messages themselves, but does not encrypt the group metadata (group name, who's in it), and does not encrypt the individual metadata (phone number, email, profile name, profile pic, etc). This means if they get a law enforcement request to give any data that is in their possession that isn't E2EE (say an individual user's profile name, groups they're in, and contacts), they can disclose that.
WhatsApp is not generally aware of the contents of individual messages or group chats - unless a user in the chat reports a message. In that case, the last five messages in the chat (reported message + 4 prior) are sent by the user's device to WhatsApp non-E2EE. Whatsapp with the report and the non-E2EE copies (made when the user hits the report button) can see those last five messages and decline to act/moderate/suspend or ban users/report to law enforcement as appropriate.
This is a key difference between Telegram and WhatsApp. Telegram is able to look at so called "private chats" (which just means they aren't searchable/have public handles within Telegram. Plenty of large Telegrams with tens or hundreds of thousands of users have Telegram invite links on public sites/public social media and are considered "private chats" by Telegram), but until Durov got arrested, they've declined to. That means if somebody on reddit posted an invite link to a Telegram group called "Stolen Credit Card Data for sale", Telegram considered it private, and even though someone from the public or law enforcement could easily join that group, see illegal activity is happening, and report it to Telegram - Telegram historically did not act on it, refusing to moderate this sort of group or worse (human trafficking, CSAM, etc.), or respond to any lawful requests for user data in such cases (even though they had the technical ability to do both).
On the other hand, Whatsapp is generally not aware of what users are sending to one another - unless a user reports. In which case, Facebook/Meta does become aware of the most recent messages, takes a look, and acts accordingly if it is either a violation of platform policy or outright illegal. For data that is within their possession because it isn't E2EE encrypted, WhatsApp complies with valid subpoenas and court orders from law enforcement.
This is why Pavel Durov is in cuffs and Mark Zuckerberg is not.