Media explorer and reaper, how to use?

Hi, so I found out that reaper has it's own media explorer build in... I was using ctrl I very time when I wanted to insert an item, but today I saw someone using it... I looked to actions and found out that ctrl + alt + x is showing it, but idk how to get inside the window... Anyone knows how to achieve that?

Just press CTRL+up-arrow until Osara reports the media explorer window, then navigate with the cursor and tab keys. Press CTRL+Alt+X again to hide the explorer.

Thx