It sounds like you’re just getting started on Bubble - rather than focusing on specific features or workflows, its really practical to approach learning Bubble like you would approach learning a new language - once you learn the syntax behind the language, using it to create specific features and manipulate your data will start to come second nature. I would recommend getting started by browsing our academy page at https://bubble.io/academy.
For your specific desired feature, the “syntax” you’ll want to learn is the basic concepts behind the workflow, design, and data tab. This video series will help a lot with that.
From there, you’ll be able to create an input for your mobile users to enter a value (assuming the “mobile app” you’re describing is part of your Bubble app), a button that triggers a workflow which sends the value to your app’s database, and then an admin page where an admin can view a “repeating group” of data from your app.
These are all core Bubble concepts, and learning the concepts rather than the feature will set you up for success.