There are many boneyards.
https://www.airplaneboneyards.com/airplane-boneyards-list-and-map.htm
Planes get sent there for various reasons, short term storage, long term storage, eventual destruction, etc.
Depending on how long they have been there and why they were sent there it can take quite awhile to bring them back into service. It is relatively common for airlines to store planes not needed until the next busy travel season. Those planes are stored ready to be brought back into service.
Many are claiming that Southwest will bring back their 737-300 that were retired as the 737 MAX 8 were delivered. It is not as simple as sending someone to the desert to fly them back home. None of their pilots would be current on that type aircraft. They would have to retrain/re-certify a number of pilots. That is expensive and time consuming. The planes have been there for two years. They may have been retired just prior to a D maintenance check coming due that will now have to be done. Expensive and very time consuming.
If the anticipated grounding is just weeks, nothing will be done except to cancel a lot of flights.
I'm interested in the training part. I heard something on the radio this morning talking about how Boeing had really pushed to make the 737 MAX certification be interchangable with the previous 737 versions, so that there was no required recertification for an existing 737 pilot to start flying the MAX. They also were able to "count" the safety record of the previous 737 versions when certifying the MAX for sale.
I have no idea how common that is for certifications, of both planes and pilots, to carry-over across versions of airplanes. Any idea? It certainly seems that if training is a contributing factor in the crashes, that the lack of recertification for this specific model is a contributing factor as well.