it's pretty counter-intuitive and anew player is not going to discover it except by random frustrated clicking; you have buttons down the bottom that sort your hot bar tabs, and these are clearly marked. Your resource bar up the top shows your action, bonus action, and class resources available - these do not look like buttons, are in a different place and have a different visual appearance to the other buttons, and give no indication that they are also clickable things - they look more simply like a resource tracker..
Just to be fair ... this idea was born here on forum ...
People wanted this ... people get it ...