First you want to either mount or unpack your ISO so you can work with the containing files directly.  Those are easy to do via the GUI using the mouse and its right-click menu.  I recommend unpacking.  Start that and walk away.  It will take some time to pack the entire disc image.

Once unpacked open your terminal and cd into the VIDEO_TS directory of your freshly unpacked files.  In there you will find some number of VOB files named like VTS_01_1.VOB.  The first of those (_0) is likely a menu item.  You can confirm this by playing it in VLC.  Once confirmed rename it from .VOB to _VOB to hide it from the next command.  The remaining (_1 through _?) should be the actual video content.  Again, confirm this using VLC.

Basically, the VOB files for actual content will be the same size except the last one which is smaller than the others.  If there are any other VOB files not part of your target content rename those also as _VOB.

Now we can merge those all together into a single file:  cat *.VOB > output.vob

(And now it’s obvious why we were hiding any superfluous .VOB files.)

Finally, use ffmpeg to go from .vob to .mkv:  ffmpeg -i output.vob  -c:v libx264 -c:a aac output.mkv

That’s the quick and dirty version which has worked for me.  You will notice it also shrinks the file size.  There is a ton of information about using ffmpeg out there so if you require different conversions, someone out there can probably guide your specific quest.