The fungus that we see above ground is the fruiting body (e.g. toadstool, puffball) that is supported by an extensive system of threads (hyphae) that gather nutrients and water.
Boletes have fleshy fruiting bodies like agarics (toadstools/mushrooms). But spores drop from tubular pores rather than gills. Polypore fungi fruiting bodies also have pores but they are generally tough and woody or leathery.
Some years ago I found a single Rhubarb Bolete (Boletellus obscurecoccineus) specimen near the Claypit but never again. Beautiful specimen.
Slippery Jack (Suillus luteus) is an imported species under the pine trees in the arboretum. It has a mycorrhizal association with pines and must have arrived with the seedlings. My fungal enthusiast mate Peter White (a good spore-t) had an Italian friend who regarded it as a delicacy, and cooked it. (I guess you close your eyes to avoid the yellow colour!). There are recipes on the internet – note you must peel it to avoid digestive problems.