Motion tracking is simple to understand, and not so simple to do. And, like a lot of VFX techniques these days, it's getting so good that you don't always know that you're looking at it
There's an example in the video below, which is a very short demonstration of the technique, but it shows in an obvious way what's been done - if not necessarily how to do it.
Look at the children's wooden blocks with the letters on them. You wouldn't know at first sight that the characters had been composited onto the bricks.
Blending it in
In a clip like this, the intention was obviously to highlight the technique. But often, the opposite is the aim. Using methods like camera shake, motion blur and distance fogging, it all helps to blend the CGI elements into the scene, in a way that's remeniscent of the way audio engineers "mix" a multitrack recording, adding reverb and EQ to make disparate elements blend in - or stand out, according to what's needed.