I add multiple videos together. The first one comes out fine but the other ones are distorted like an old vhs tape messing up.
Are you re-encoding or using copy mode ?
If copy mode, what codec are the input videos encoded with ? If it's x264, do the videos have the same encoder settings ?
I was using copy mode, re-encoding mode doesn't have this issue. The only issue is re-encoding mode renders a lot slower.
Sure. Re-encoding also degrades quality at constant bitrate.
Joining files without re-encoding isn't straightforward if the files don't have the same encoding settings. If the files are from different sources, they are likely to have different encoding settings.
Bits of information missing:
1. Avidemux version (should be at least the latest release – 2.7.8 or better the latest nightly build (https://avidemux.org/nightly/)) and platform.
2. Properties of the first and subsequent source videos as listed by MediaInfo.
As already mentioned by butterw, for successful merging of videos in copy mode, video codec and codec settings of all clips as well as properties of all audio tracks (audio codec and codec settings, sampling rate and number of channels) must match perfectly.