It can be next to impossible to manually search a Telegram channel for one user's messages, so you will want to use the Search tool. From within a channel, tap on the title of the channel at the top center to reveal some tools. Tap on the Search tool.
If you have joined a Telegram channel with 20K users you are going to want to mute it. In fact, you probably want to mute it if it has 20 users. To do so, simply swipe left on the channel you want to mute under the Chats tab, then tap on the Mute button.
For those not familiar with it, Telegram is a popular messaging app with privacy features such as end-to-end encryption on "secret chats" that can only be accessed on the devices they originated on. Because of this it has gained notoriety as a tool used by terrorists and has even been banned in Russia and Iran where it has been deemed a tool of social dissidents. Nevertheless, Telegram is a well-done app for simple, fast and secure messaging.