When layering lots of strings it’s a good idea to very slightly de-tune them against each other, and introduce a little bit of pitch variation in what each individual instrument is playing as well.
The violin family, having no frets, tend to wander a tiny amount in pitch both between players and how exactly each player fingers and bows each note. That, along with a bit of vibrato, again at slightly varying speeds, is what gives a string section its “fatness”. Though traditional folk fiddlers and baroque music specialists tend to use less vibrato than the modern orchestral players.