In Arduino IDE 2.0, the Serial Monitor does not open in a popup window. Rather, it opens in a new tab at the bottom, besides the Output tab.
The Serial Monitor can be accessed by Tools → Serial Monitor, or using the keyboard shortcut (Ctrl + Shift + M)
The following figure highlights the various parts of the Serial Monitor.
Note that because the IDE 2.0 is in the beta mode right now, the Serial Port may not be detected immediately. In some cases, you may need to restart the IDE for it to be detected.
Similarly, the Upload button may not work sometimes. You can use Sketch → Upload in such cases.