well, i kind of just answered my own question with a little more googling:
'While Toyota’s team worked on battery and powertrain, Subaru handled chassis, AWD, and crash systems"
taken from a car&driver writeup:
https://www.caranddriver.com/news/a3...erra-revealed/
so i guess the solterra is split a little more evenly between the two manufactures than the frs/brz