Currently we have Teams defaulted to show you the first (up to five) channels ever created. If you want to add more channels to your left rail view, just click the ellipses next to the channel name and select “Favorite”. When you favorite the channel, it will then be “pinned” to your left rail under that team name.
Try that and let me know if that meets your needs!
This favorite feature is utterly useless when you are in a company where the channels can change weekly or sometime daily. I just want to see all the channels on the pan and be able to scroll down as needed. If you have certain few you use all the time you can favorite them to be on top of the list just like I do on Microsoft Outlook.