Usually what happens are major storms are reported with POSSIBLE tornado rotations.

Then the meteorologist will watch the storm like for possible cold/hot air rotations within the front line of the storm. (I.E. Texas getting Northern cold air fronts that meet with the Gulf coast hot air)

If they see anything within a possible amount, theyll put out a Tornado WARNING. Which means "Hey, the air is getting funky and one can spawn easily."

Then usually storm chasers/trackers will report tornado touchdowns or activities to certain channels that will enable the Tornado WATCH. Essentially someone called it in and they'll watch the physical tornado plus storm line to guesstimate it's path.