I had a 595mb .MP4 video (data rate: 510 kbps, Total bitrate 638 kbps). And I joined it with another .MP4 youtube video with a lower quality(only the end part).. The resulting video file has a data rate of 389kbps and Total bitrate of 517 kbps.. The size is also significantly smaller; 494mb.. I used a program called Video Combiner.. And side by side I can see the original one is better quality looking.. so what am i doing wrong? any suggestions? I just wanna join 2 video files without loss of total quality.
Your program probably re-encoded the resulting video file from scratch and used whatever setting it considered okay.
If you want to have more control about the process and/or do not want it to re-encode the video at all (if both are of the same format), then I strongly suggest looking into ffmpeg for this.
For reference, https://trac.ffmpeg.org/wiki/Concatenate - the concat demuxer is what you want.
As the most simple example, copy both your video files (let's call them videoA.mp4 and videoB.mp4) into the same folder. Create a videos.txt file with just this content:
file videoA.mp4 file videoB.mp4
Now run ffmpeg with this command:
ffmpeg -f concat -i videos.txt -c copy videoOut.mp4
If all goes well this gives you a new file with both videos in one file, with the original encoding intact.