Anything that runs as long as you want the block to be. Usually sleep is a good one, use sleep infinity if you want the blocker to never go away until the systemd-inhibit process is killed manually.
There is kde-inhibit --screenSaver <command> provided by KDE.
But these days, I would just recommend everyone to use systemd-inhibit --what=idle --why=<because> --who=<myself><command> instead. Works across desktops and does the same thing.
Presumably, but it prints "y" to the program output as fast as your CPU allows it, so that's probably not a very efficient way to do it.