Doubts..mask and monitor transparency

Hello!
Have two doubts with this video...
1. When you crop side of a video, Can you see the monitor with transparency instead of black background?
2. I want to add a mask to whole V2, not only afecting video, for all videos that i will place in v2. I tried to look for a tool for making a mask but didnt found. I tried too to put on V3 a video for using as a "track matte" but didnt found (again) how to link to v2...
Can you explain me how to apply a mask on edit mode?
Regards!
Have two doubts with this video...
1. When you crop side of a video, Can you see the monitor with transparency instead of black background?
2. I want to add a mask to whole V2, not only afecting video, for all videos that i will place in v2. I tried to look for a tool for making a mask but didnt found. I tried too to put on V3 a video for using as a "track matte" but didnt found (again) how to link to v2...
Can you explain me how to apply a mask on edit mode?
Regards!