To unlock this sidequest, you have to upgrade your Sheikah Sensor in the sidequest "Slated For Upgrades". Learn more about it here.
Take a picture of the Sunshroom behind the lab. Note that Symin will only take a picture of a Sunshroom taken in that location. If you already picked it, then drop it on the ground to take a picture of it where the Sunshroom originally grew.
Now Symin will tell you to activate the Sheikah Sensor to respond to Sunshrooms. He wants you to gather three. So go to your Hyrule Compedium and go to the Sunshroom page. Press [A] to activate the sensor, and scour Hyrule in search of the Sunshrooms based on your Sensor. This serves as a tutorial to show you how you can make the most of your upgraded Sensor!
Once you give Symin three Sunshrooms he will give you three Hearty Truffles in exchange. You can use these to artificially extend your health when cooked so the trade is well worth it.
Additionally, Purah will let you activate her Guidance Stone out of curiosity and you will be given a random Hyrule Compedium photo that you didn’t have yet. From now on, you can pay 100 Rupees to fill out your compedium. Note that certain photos you have to get yourself or are not purchaseable until you have beat the game at least once.