It's been a while since I was here... So, I have this question: I want to make a video on a two-level map (of course, the underground level is created), but in the dialog scene editing window, I can't switch to the underground. How can I do that? And another question, for the future) How to play effects during a video in the underground. What height should I set?

If you need to shoot on two levels (both on the surface and in the Dungeon), then you will have to design part of the surface as a Dungeon. Fence off part of the map with underground walls. You can see an example in the dialog
DialogScenes\A2Single\SM2\S2\DialogScene.xdb

Added 2 minutes ago
The second question then becomes irrelevant.

Added 7 minutes ago
If the surface is not needed, then
1. Open the map with Notepad
2. Change the InitialFloor parameter to 1.
3. Close
4. Save

When opening the map/dialog, the Dungeon should be displayed.

Then set the height to ≈17