$WW,1$The editor mostly stays in a $LK,"GetKey",A="MN:GetKey"$()/$LK,"PutKey",A="MN:PutKey"$() loop.  The putkey portion is where keys are acted-upon.  TempleOS has a chain of putkey handlers in a $LK,"Circular Queue",A="HI:Circular Queue"$ with priorities.  The highest priority handlers can choose to terminate handling, otherwise, the keys get sent on down the chain.

$LK,"KeyDevAdd",A="MN:KeyDevAdd"$() defines a putkey device with a priority.  "Device" might be a misnomer.  Currently, the following are defined:

Priority    Handler
---------- ---------
$FG,2$0x20000000$FG$ $LK,"MyPutKey",A="MN:MyPutKey"$() user handler
$FG,2$0x40000000$FG$ $LK,"KDInputFilterPutKey",A="MN:KDInputFilterPutKey"$() for $LK,"Auto",A="MN:Auto"$(), $LK,"AutoStr",A="MN:AutoStr"$(), and $LK,"AutoFile",A="MN:AutoFile"$() handling.
$FG,2$0x60000000$FG$ $LK,"KDRawPutKey",A="MN:KDRawPutKey"$() nonwindowed direct to video mem debug output. 
$FG,2$0x80000000$FG$ $LK,"KDDocPutKey",A="MN:KDDocPutKey"$() standard document cmds

Since handling individual keys is slow, TempleOS supports PutS() as well.  If no puts handler is defined, individual keys are sent.

$LK,"CDoc",A="MN:CDoc"$$FG,2$.user_put_key$FG$ and $LK,"CDoc",A="MN:CDoc"$$FG,2$.user_put_s$FG$ are call back routines which offer some neat tricks.  See $LK,"::/Apps/Psalmody/JukeBox.CPP"$.  There is a var $LK,"CDoc",A="MN:CDoc"$$FG,2$.user_put_data$FG$ which gets passed to them.