The modern site is a template to accommodate your growing video library. Administrators add a site collection that incorporates multiple video channels.
If you access the modern site while you are using your browser in full-sized mode, then the channels will automatically listed on the left-side of the screen.
If your are not using your browser in full-size screen mode, you will have to click on the hamburger icon for the list of channels to appear.
Then select the channel you'd like to change to.