This new model will be built on the on the e-Subaru Global Platform, but technically it's a platform developed with Toyota, which calls its platform the e-TNGA. It's the same thing as both companies have announced in 2019 partnership on both the platform and the all-electric SUV.
"The e-Subaru Global platform, which has been jointly-developed by Subaru and Toyota, enables to roll out various types of EVs efficiently by combining multiple modules and components, such as the front, center and rear of a vehicle. While the two companies have brought together their knowledge to create a completely new platform, Subaru have aimed to realize superior passive safety and vehicle stability, which Subaru has always been pursuing, also in this BEV dedicated platform."
If we compare the Toyota bZ4X concept with the Subaru Solterra, it turns out that the two are pretty much identical and only design details are different.
A joint project like this is a major boost that allows to reduce costs and accelerate development. The following models might be more diversified, but considering the ongoing global consolidation in the automotive industry, nothing is certain.
Here is an unofficial video that shows the Subaru Solterra in different colors: