Villagers don’t turn into a farmer through the composter. At one point it can be made by breaking the profession block.
In some cases, they do not turn into farmers even after breaking the professional block.
Even after leaving a composter with the villager which was the only professional block at the spot, they do not change professions.
The villager also refuses to do his farming job if his profession is not completely changed to farmer.
In the same way the villager does not grow stuff, harvest stuff or do something that a farmer can do.
This issue mainly occurs due to some irritating bugs.
Hence the only way for changing a villager into a farmer is to have unemployed villagers. The option for changing villagers into farmers is entirely random, you have to look for the adding unemployed villagers.
Likewise, after adding the unemployed villagers if the composer shows green particles, that means the unemployed villagers have turned to farmers.