DVD-Video disc has common directory and file structures. The root of the disc has a directory named VIDEO_TS, which contain a bunch of files with names ending in IFO, BUP and VOB. The large VOB files contain the actual video content, while other files contain information about the video menu, video file structure and sequence.
As video in DVD split on a few parts (which are stored in VOB files). To convert whole DVD disc into a single file you should convert VOB files one by one into your desired format and find an utility, that will merge these files for you later.
We recommend you to use https://mergevideo.online/ to accomplish this task.