Here is how to join media files with ffmpeg.
Create a file mylist.txt with all the files you want to have concatenated in the following form (lines starting with a # are ignored):
# this is a comment
file '/path/to/file1'
file '/path/to/file2'
file '/path/to/file3'
Note that these can be either relative or absolute paths. Then you can stream copy or re-encode your files:
ffmpeg -f concat -i mylist.txt -c copy output