When you're online, the Profile Banner is replaced by your live stream. The Profile Banner only shows up when a creator is offline. When you enter the 'Customize Channel' area, you will see this default 'Generated Background' option selected. By default, a Twitch channel gets a banner which shows your username repeated in semi-transparent letters. The Profile Banner shows up behind your channel name, heading, and video box at the top of the profile page. Part of your channel build includes the Twitch Banner, which can be tricky to size and design. Twitch dominates live streaming, so if you’re looking to build a live channel, it's the place to be.
Twitch recommends a 1200x480 banner size – I'll show you how to format your Twitch banner precisely, using Kapwing.