Why did the stay at home order come from a judge and not the mayor?

I was curious why the judge hildago had authority to issue the order and not the mayor? I’m all for it and not complaining I’m just curious how the authority on it works. I understood judges to be the arbiter of disputes between individuals (civil) and the state/individual (criminal). Was there a law or order issued from the state or federal level that was being interpreted or do I misunderstand something about judges?(I’m sure I do).

Can the governor or a mayor issue a similar order? Or only judges? Is it because this is Texas and we just decided only judges can do this and in other states it is an executive thing? I’ve searched online and all I found was mess articles saying a judge did it but not why they can.