Making a collage of several videos
Blackmagic Design’s DaVinci Resolve offers a free version, is available on Mac, Linux, and Windows, and is well-suited to creating video collages. One of the effects added in version 17 is actually called “video collage” and greatly simplifies the process over past versions.  Additionally, if you synchronize the timecode of all cameras before you start shooting a multi-camera event, it offers features like “live overwrite” and “sync bins” which simplify typical multicam edits.
Adobe Premiere is a viable alternative, but does not run on Linux, and is not free.
Final Cut Pro X is a Mac-only alternative, and also does not offer a free version.
Avid Media Composer is the other big-name player in the NLE space.