An app is just computer code. It doesn’t “get tired,” “wear out,” or suddenly “break down” — so once launched, it requires no additional maintenance.
The only ongoing cost is paying for the server (hosting).
You pay a subscription fee for the server. The more users your app has, the more server resources it consumes, and the higher the hosting costs.
At the early stages, when you have few users, the server will be FREE.
After that, the cost will gradually increase, but it remains affordable.
For example, an app with 100,000 users spent about $150 per month on hosting.