I was looking for software to more easily split video files (which my video conversion software can also do, but it's very unwieldly) and join video files (which my conversion software can't do at all). ANd I ran by this free software that appears to be discontinued so now it's free. The software is a trial download, but they give you a the registration keys for free.
AVI, MOV, MKS, WMV, etc. The extension on the file is the container format. It's completely independent of the codecs used on the audio/video data itself.
The only tool I know of that will do everything is ffmpeg, but it's command line only and has a much higher learning curve due to it's enormous feature count.
The one place you'll start running into issues is if the codecs don't match between the files. Then you have to do re-encodes to make everything the same. But as long as the same codecs are used in all the files, there should be no problem putting them back together.
MPEG will actually be a little easier, as it specifies the container and the codec.
The only tool I know of that will do everything is ffmpeg, but it's command line only and has a much higher learning curve due to it's enormous feature count.