Water damage floor

I know there are numerous threads in this but I feel helpless so any advice I can get specific to my issue is greatly appreciated. Noticed the bathroom floor getting soft. Removed the toilet hoping it was the seal, doesn’t seem to be the seal but I could tell the floor was pretty bad. I removed the vinyl and found it is very bad and extends from the red arrow all the way under the shower. The hole you see there, I was able to push my hand through very easy. Past the bathroom wall and into the kitchen is very solid. Under the bathroom sink is very solid as well. Problem is I can’t tell where the leak is coming from. I can probably handle a repair of removing the shower, repairing the subfloor, but if the us a problem inside the walls I think I’m screwed. Ugh.