I'm getting some unknown requests in my "app engine" aplicattion and want to know what they are

Question

it's just a simple express js web app in standard environment, i put my custom domain and is working ok, but a couple of days ago I started receiving these requests and I want to know what they are.

Answer 1

Those requests are typically Let's Encrypt requests to verify that you control the domain during the SSL certificate validation process.

The response is 404 Not Found which means the pages do not exist.

Those requests are harmless provided that you do not receive a massive number of requests.