有些事如何做: How can i link between two applications

I am new to programming and now I am working on a restaurant app through flutter. I have a problem, which is I created a user app and also created a restaurant owner app in a restaurant owner app. There will be an addition to the meals that he has. My question is how do I link the two apps so that when the restaurant owner adds the meal, the meal appears in the user's app. I hope for help