Saturday Dual GM Planescape 5e
Doors Across the Net
This is just a list of various sites you can go to learn more about Planescape.
The preeminent fan page for Planescape. The forums are active with a lot of knowledgeable people. The site is going through some transition due to a server crash so a lot of the old archive material is being slowly shifted over, but most of that is available through an online backup version of the old site.
An old but still well maintained website with a ton of Planescape lore. A great place to start if you’re looking to look something up or just browse and learn more about the setting.