If I remember correctly, for ZFS it was some stuff about cache size and RAM usage, since we were in a VM and couldn't be too greedy. Something about a NVME drive needing slightly different settings. I didn't spend much time on this. For Postgres it was so so so many things, I played with it on and off for months, benchmarking certain key queries and such. And I read a LOT of blog posts about how to make Postgres and ZFS work together ideally, one thing I remember in particular was waffling back and forth about logbias=throughput vs logbias=latency. Just google "postgres zfs logbias" and you'll get MANY conflicting opinions. (edit: when I google it now in incognito, my reddit post is at the top lol)

But for what I actually changed, off the top of my head, I set the recordsizes in ZFS large enough that Postgres could safely (because of ZFS CoW) have full_page_writes off, and combined with synchronous_commit off, that really sped up the overall system and made the WAL logs much smaller. After looking just now at postgresql.conf, various other things were tweaked, such as seq_page_cost, random_page_cost, effective_cache_size, effective_io_concurrency, max_worker_processes, default_statistics_target, dynamic_shared_memory_type, work_mem, maintenance_work_mem, shared_buffers, but those were not quite as important. (plus some uninteresting tweaks to WAL behavior, since we had replicas that got WAL logs shipped every few minutes with rsync)