~11 min
An unemployed villager takes on a profession by claiming the nearest job site block it can reach: a lectern makes a Librarian, a blast furnace an Armorer, a smithing table a Toolsmith, a grindstone a Weaponsmith, a brewing stand a Cleric, a composter a Farmer, and so on for each profession. Once that villager completes its very first trade, the profession and its offered trades lock in for good — breaking the job site block afterward doesn't undo anything. Before that first trade, though, breaking and replacing the job site block rerolls what it's offering, which is the standard way to fish for a specific trade.
Villagers climb through five trade tiers as they gain trading experience: Novice, Apprentice, Journeyman, Expert, and Master, with each new tier unlocking further trade options on top of the ones before it. Every trade uses emeralds as its currency, whichever profession is involved. Emeralds themselves come from emerald ore, found only in mountain biomes and requiring at least an iron pickaxe, or from trading itself, since plenty of villager trades pay the player emeralds rather than take them.
Prices aren't fixed once a trade unlocks. Demand is tracked per item: trading the same item repeatedly, or emptying it out entirely, pushes its price up the next time the villager restocks, while leaving a pricier trade alone for a while lets it drift back down. Reputation works alongside demand as a separate lever — trading with a villager over time, defending a village from a raid (which grants the temporary "Hero of the Village" effect), and curing a zombie villager all build positive reputation and cheaper prices, while in Java Edition hitting or killing villagers raises them. Curing a zombie villager is the strongest of these: it gives that specific villager a permanent discount, plus a smaller temporary discount that spreads to other villagers nearby.
No — repeated trading raises that specific item's price through demand, so the sixth purchase will likely cost more than the first.