Chats are not e2e encrypted by default, they are just encrypted in transit. However this allows chats to be synced across many devices, so it is very very convenient.
Telegram has e2e encrypted chats but only on mobile and not on desktop for some reason.
telegram is e2ee only for secret chats, all other chats & group chats are not e2ee (which means telegram can access their content at will on the servers)
Synced chats across devices is possible with e2ee, even signal has this, it's just one edge that's poorly implemented: initial sync of the chat history and afaik they haven't fixed this yet, but all messages after setting up a new device are in sync as far as i know
Telegram has e2e encrypted chats but only on mobile and not on desktop for some reason.