Telegram's E2EE isn't available for group chats. It's not on by default for other chats, so most or all of your chats are probably just transport encrypted. Further, they rolled their own crypto (bad), MTProto2, which has a number of problems (but is not necessarily broken)

This places Telegram's security stance below that of even Instagram or Facebook (which also has optional E2EE chats, but uses the Signal protocol, which is considered better than MTProto2.)