In Arduino IDE 2.0, the boards manager is present in the navigation panel on the left.
It can also be accessed using Tools → Board → Boards Manager.
If you open the boards manager, you can see that it allows you to download packages or cores, each containing one or more boards. For instance, instead of downloading only Uno, you download the 'Arduino AVR Boards' package, and this includes several other boards (like Mega, Leonardo, etc.) apart from Uno.
Search for your board of interest, and click 'Install'
The progress can be seen in the 'Output' tab at the bottom.