The autocomplete feature is a welcome addition in Arduino IDE 2.0. This addresses a major drawback of Arduino IDE when compared to other IDEs, like Eclipse of VS Code.
In order to use the auto-complete feature, you first need to select your board (this feature doesn't work till the board is selected). Once that is done, the suggestions should start popping up as soon as your start typing characters, as per the documentation.
However, in some cases, like on my machine, this doesn't always happen. In such cases, you can click Ctrl + Space for the suggestions to appear. Perhaps that is because this IDE is still in beta mode.
I should note that this feature isn't as robust as you would want it to be. For example, sometimes, Serial.p gives no suggestion, whereas I had expected suggestions like print, println, etc.
Perhaps these fixes will be made in the beta version, and the feature will be quite robust by the time this IDE is launched in production.