The lead and second violin parts don't sound particularly good to me (as in "realistic"); kind of crappy, actually. The section stuff sounds pretty good (if you mean the little string breakdown at about 2:05). But these aren't what I'd call "exemplary string sounds" by any means.
The trick to any of this stuff is using the desired instrument sound in its actual range (meaning, don't play it really high or really low if that's not what the actual acoustic instrument can do), and also, importantly, trying to "think"/play like a violin player (or trumpet, or French horn, or whatever). That involves examining how the acoustic version of the desired instrument sounds, and what the basic playing techniques are. So if you're trying to emulate a violin, listen to some violinists first. If you want to write convincing string section parts, listen to some records with string sections and see what the arranger wrote, and listen for how the section players interpreted it.
For example, in the video above, I think the violin melody is much too choppy, and there really isn't any vibrato applied at all. Even a staccato melody like that would have some sustained notes and a bit of vibrato in there, unless it was played pizzicato (no bow; fingers only). Adding those things would add a layer of realism.
When you're thinking about range, also think of that in terms of parts and arrangement. If you want the strings to stick out a bit, then you don't also want to have organ, piano, and guitar all playing the same chords as well, using the exact same voicings in the exact same register. A C chord on top of a C chord on top of a C chord on top of a C chord, all playing triads of C-E-G, will just add up to mud when it comes to mix time. So those things have to be thought-out a bit ahead of time.
But I digress. To make more realistic strings, play them like a string player would. This will require some listening and analysis.
GJ