This page explains how to start and complete the Sunshroom Sensing side quest.
How to Start the Quest
This quest is unlocked as you progress through the Locked Mementos quest. You will need to have Purah upgrade your Sheikah Sensor, then talk to Symin to start the Sunshroom Sensing quest.
How to Complete the Quest
Go out through the door near Symin. There is a Sunshroom nearby. Take a picture of it with your Camera rune. It will be added to the Hyrule Compendium. Go talk to Symin and tell him you took the picture. Then he asks you to collect three Sunshrooms for him. He says that they can be found in Retsam Forest, which is just north of the lab. Press - to open your Sheikah Slate, then press R to go to the Hyrule Compendium. Select the entry for Sunshroom, then press A to target it with the Sheikah Sensor. Then glide down to the forest and collect the Sunshrooms. Then you can warp back to the lab and talk to Symin to complete the quest.