How to find and delete multiple videos

To add to the good advice to set the Library filter for video, before doing that set the source by selecting one or more folders in the Folders panel, not anything in the Collections panel. The reason is that if you press the Delete key when a folder is the source, an alert will appear that offers to delete the actual file, not just its entry in the catalog database.

I have triedd selecting 'collections' but it does not let me delete!

By @Terence52

If the selected source is in the Collections panel, it won’t offer to delete the actual file because collections are virtual lists, like a playlist in a music app. Delete removes only the file’s entry in that collection, not the file itself.

There actually is a way to delete actual files when viewing a collection, but it involves a keyboard shortcut I haven’t memorized and it does it without an opportunity to confirm. So it’s safer to delete original files while viewing a source in the Folders panel.

Click on "All Photographs", select Grid view, then use the Library Filter Bar (press the back-slash key "\" to expose the Bar if it's not being shown). Choose File Type for the first column and you can then click on the Video entry. That populates the grid with all your videos, and you can select and delete them from there.