This page details general resources, other pages have embedded links related to specific topics. If you are working on your own car the Subaru Factory Service Manual (FSM) is generally a good document once you get used to the layout. For US & other markets PDF files are available with web versions becoming available for later Japanese market models. Sometimes the procedure's described are a little cautious but thorough nevertheless also the diagrams don't show quite enough detail but not a big problem.
There's also official technician training material and technical service bulletins (TSB's). Here's some links for these resources and how to get hold of them.
Depending on your region you can use an online portal provided by Subaru to search recall information by model, VIN or license plate.
Your car uses many computers to operate - occasionally there are updates made available by Subaru. This Subaru document lists engine, transmission, vdc software updates available for each model and year. Normally only the dealership can install these, usually for a fee. It's worthwhile having the dealership check for updates before beginning major troubleshooting efforts.
If you prefer genuine original equipment manufacture parts buying from the dealership can be expensive. Subaru's tend to be fussy and prefer genuine parts.
Both amayama and partsouq help you search for matching parts based on your vehicles chassis number (JDM models) or VIN (other markets).
There's too many to mention here's some of the notable consistently used forums.