So why are Bilbo and Frodo not spending all of their time on fields? Why are they not starving when coming back? What do the other hobbits trade food for, is there some trade for metal implements or something?
Because they are landed gentry. The Gamgees and other working class hobbits do the field work. Bilbo was wealthy even before his adventure. Pippin and Merry are also members of the aristocracy. Farmer Maggot was something more or less like a yeoman farmer.
The Shire is, in basically every respect, including its economics, an idealized version of the English countryside.
Apart from Sam, who is a peasant, Gandalf, who is a demigod, and Frodo, who is landed gentry with close familial ties to the local aristocracy, everyone in the company is an aristrocat: Legolas is the son of a king, Pippin and Boromir are heirs apparent to the local representative of the absent king, Aragorn is pretender to the throne of said king, Merry is heir apparent to the second most important local ruler after Pippin's father, and Gimli is a member of Durins house.
