I've worked on quite a few large-scale scientific collaborations like this (and also worked on/talked to the lead scientists of LSST) and typically, the end groups that do science aren't the ones handling the massive infrastructure. That typically goes to well-funded sites with great infrastructure who then provide straightforward ways for the smaller science groups to operate on the bits of data they care about.

Here's the canonical example: https://home.cern/science/computing/grid and a lab that didn't have enough horsepower using a different grid: https://osg-htc.org/spotlights/new-frontiers-at-thyme-lab.ht...

Personally, I have pointed the grid folks (I used to work on grid) towards cloud, and many projects like this have a tier 1 in the cloud. The data lives in S3, metadata in some database, and use cloud provider's notification system. The scientists work in adjacent AWS accounts that have access to those systems and can move data pretty quickly.