Minecraft villagers have their own professions, but these positions aren’t necessarily permanent. Players can make their villagers take up a different job.

Villager professions in Minecraft are attached to their job site blocks. Without access to these blocks, villagers essentially remain unemployed.
Removing a job site block will also remove its connected villager’s profession. However, if another job site block is within roughly 48 blocks of the newly-unemployed villager, they will attempt to claim it and find new employment.
Minecraft: Changing villager professions and knowing job site blocks
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As long as the unemployed villager claims the block before any others, it will change its profession successfully. This can be repeated as many times as the player wants.
It’s worth noting, however, that a villager’s progress in a given profession is not preserved when it changes. Therefore, players may not want to change the jobs of their most experienced villagers. Instead, wait for novice-level or newly-bred and unemployed ones to come along.
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