Couple of links that should help with Context Memory.
https://nodered.org/docs/user-guide/context
https://discourse.nodered.org/t/a-guide-to-understanding-persistent-context/4115
TBH, I just set up a little Noddy Flow to see what happened and viewed the Context Data window on the RHS. Easy once you know how (says I! 
).
The same for MQTT, install a server on the PC with Node-RED - I use Mosquitto, again the words are more difficult than the doing. Remember, MQTT Explorer will help initially, and once you can see what your device is sending, then you can check to see what you can read in Node-RED.
