For video files that come in part, use ffmpeg to merge them into 1 file
Eg: part1.mp4, part2.mp4, part3.mp4
Create a concat file - list.txt
file 'part1.mp4'
file 'part2.mp4'
file 'part3.mp4'
file 'part2.mp4'
file 'part3.mp4'
Use ffmpeg to merge into full.mp4
ffmpeg -f contact -safe 0 -i list.txt -c copy full.mp4
If you have subtitles create an additional concat file - subts.txt
file 'part1.mp4.srt'
file 'part2.mp4.srt'
file 'part3.mp4.srt'
file 'part2.mp4.srt'
file 'part3.mp4.srt'
Then use ffmpeg to create the full file
ffmpeg -f concat -safe 0 -i list.txt -f concat -safe 0 -i subs.txt -map 0 -map 1 -c copy -metadata:s:s language=eng -metadata:s:s title=English full.mp4
The title= seems to be necessary when doing subtitles and it names the subtitle track.