My previous startup (~2014) had a similar problem: PBs of data, with millions of mixed clients accessing it at close to real-time speeds. The biggest difference is that we needed to do real-time processing before delivering the content. We needed storage capacity balanced with CPU and RAM.
We ended up buying lots of Supermicro's ultra dense servers [1]. That's a 3U box, containing 24 servers that are interconnected with internal switches (think: 1 box is a self-contained mini cloud). Each server has (cheap config) 1 CPU 4 Xeon cores, 32GB ram, 4TB disk.
Those were bought & hosted in China, and IIRC price tag was around $20k USD per box. That's 96TB per 3U, or >1.2PB and ~$200k per rack. We had a lot of racks in multiple datacenters. These days capacity can be much larger, e.g.: 6TB disk, 144TB per 3U and >1.8PB per rack.
We've tried Ceph, GlusterFS, HDFS, even early versions of Citus, and pretty much everything that existed and was maintain during that time. We eventually settled on Cassandra. It required 2 people to maintain the software, and 1 for the hardware.
Today, I would have done the same hardware setup, mainly because I haven't had 1 Supermicro component fail on me since I bought them first in early 2000s. Cassandra would've been replaced by FoundationDB. I've been using FoundationBD for awhile now, and it just works: zero maintenance, incredible speeds, multi datacenter replication, etc.
Alternatively, if I needed storage without processing, but with fast access, I'd probably go with Supermicro's 4U 90 bay pods [2]. That'd be 90*16TB, 1.4PB in 4U, or ~14PB per rack. And FoudnationDB, no doubt.
As a fun aside: back then, we also tried Kinetic Ethernet Attached Storage [3]. Great idea but what a pain in the rear it was. We did however have a very early access device. No idea if it's still in production or not.
[1] https://www.supermicro.com/en/products/system/3U/5038/SYS-50...
[2] https://www.supermicro.com/en/products/system/4U/6048/SSG-60...
[3] https://www.supermicro.com/products/nfo/files/storage/d_SSG-...