Sometimes the vendor will provide an optional fix in a safety information bulletin, sometimes they will be mandatory (Sometimes the optional ones become mandatory [1]).
They are coordinated with applicable certification bodies (civil aviation authorities) and distributed as airworthiness directives that can, in fact, force a specific action to be taken.
[1] (writing from memory unfortunately) an airflow modification for 737 NG (iirc, could be older 737, pre-MAX definitely) avionics bay was "optional", as in mandatory only for aircraft flying in hot enough regions. After a near miss in Poland when steadily overheating avionics essentially slowly lobotomized a plane after takeoff. Turned out europe got hot enough for it.
After that incident, Boeing issued a change in safety information bulletin that the modification was now mandatory.
They are coordinated with applicable certification bodies (civil aviation authorities) and distributed as airworthiness directives that can, in fact, force a specific action to be taken.
[1] (writing from memory unfortunately) an airflow modification for 737 NG (iirc, could be older 737, pre-MAX definitely) avionics bay was "optional", as in mandatory only for aircraft flying in hot enough regions. After a near miss in Poland when steadily overheating avionics essentially slowly lobotomized a plane after takeoff. Turned out europe got hot enough for it.
After that incident, Boeing issued a change in safety information bulletin that the modification was now mandatory.