Everyone's hand is a different size and shape, so following videos sometimes has to be done thoughtfully, and it makes forum discussions problematic. One book says that your thumb is "by far" your longest digit (the thumb, just like any digit, has three bones, and doesn't stop until you get to your wrist). But I've just measured mine and it's the same length as my middle finger. The main thing is be comfortable, and the thumb should be so relaxed that you can tap it on the neck of the violin if you pause your playing. My teacher has a little felt duckling she uses on kids to get a small gap between the neck and the yoke between the thumb and the first finger.
The downward force from the weight of the neck should be supported on your thumb's middle bone (I think there may be an Itzhak Perlman video on that one), but it's tempting to support it lower down on the mons thingummy.
I find my thumb has to be quite a long way towards the bridge for my pinky to be able to reach what it has to reach, whereas Pierre, from memory, likes to have his thumb closer to the scroll (forgive me if I'm wrong). I position my hand by playing B on the A and E strings, positioning my thumb then doing a descending B major scale starting on the A string and verifying that my pinky can reach A# on the D string. Of course remembering to keep my left wrist straight interferes, and vice versa. I have a few pieces of music where I have written "thumb" in pencil underneath certain passages - this is because my thumb has a tendency to drift too far towards the scroll for what the music requires from my pinky.