Most people never wonder.

Something you should know before you convert any videos is that most converters will just re-encode by default even when they don't have to.

Re-encoding is decoding and re-encoding every single frame, and only doing it once may not be too bad. But doing it multiple times, especially on the same file, can add up and really hurt the quality, like getting a nice picture on your phone, but it looks blurry when you finally get it on the TV.

The real way to avoid that is with remuxing, which is basically just putting the video and audio in a different container, which takes almost no time, uses no quality, and is called -c copy in FFmpeg, but this is only possible if the codec is already supported for the target format.

Now what you should really ask yourself when you are comparing video converters is:
Does it remux when possible or does it always re-encode?
Does it keep the frame rate, ratio, metadata, and audio stream?
Can it batch process all of your files together if they are a different format instead of making you do each one separately?
Do they have a limit on file size?
I'm currently testing out Kernel Video Converter on some mixed formats, and it claims to support MP4, MKV, MOV, AVI, MXF, MTS, WEBM, WMV, DVD VOB, and many more for the input videos. You can convert them to AVI, MP4, MKV, MOV, extract MP3, M4A, WAV, AAC, OGG, WMA, and AC3 audio, keep the frame rate, ratio, metadata, and audio stream for HD conversions, can batch process mixed formats all together in one process, and doesn't have a limit on file size that I could find, but it is only available for Windows.

One thing to note about Kernel Video Converter is that it isn't free, but the trial lets you convert anything you want as long as you only view the first 15 seconds of the converted video, which should be enough to see if it works for your needs before you buy.

Has anyone else here ever bothered checking a converted file's bitrate to see if it looks okay, or do you guys just assume that it is fine because it looks okay?
https://www.nucleustechnologies.com/video-converter/