How do you get 9999 sp so fast

Having a Blessing conferred helps double SP gained as a good starting point.
I also tend to play with the unit I need more SP with a lot and tend to ignore spending it for a while, particularly on the weekends or when TT comes by.
Valor skills on Special Training maps are also good for grinding, though those are hard to come by since they're mostly locked to 5 stars.

You either use them a lot with Valor skills or you abused that glitch that maxed out your SP almost instantly. I've done both.

The few characters I have that have max SP are the ones I use almost all the time. I barely notice until they've reached it.

Unless you're talking about reaching high SP to build a unit. It's hard to reach max SP when your unit's build isn't complete.

Most of what I see posted look like unit builder, and those previews have maxed out values.

But like said above: a blessing, a valor skill, and a double xp/sp weekend can net 24sp per kill. 48sp if they're a focus unit for a tempest trial.

Some of them might be from the Heroes unit builder. The display defaults to max SP and HM. Otherwise, you'll get a lot of it from Tempest Trial grinding, regular mindless grinding or by using the unit a ton.