I am not sure if I understand your question but hopefully I can answer. Every user will have Runtime installed on their machine with a package loaded. Runtime can start a browser session and enter a URL and see when controls are clicked among other things. It can also track when a browser session is started and a specific URL is entered... in both scenarios, as long as you have the button interrogated in your solution, you can create an automation that begins when that button is clicked.
The deployment portal is commonly also called OMC. This is a cloud-based service that allows you to upload packages too and then assign it to users. When Runtime starts on their machines, Runtime will reach out to OMC and check for a package to load and automatically start this. OMC also automatically installs the version of Runtime that is assigned to the user (User 1 assigned package A with Runtime v8.0.1018 and User 2 assigned package B with Runtime v8.0.1053).
The deployment portal is completely optional... you can just as easily manage your packages yourself and deploy them to users and simply install the runtime version required.
Thank you very much for the details of Deployment Portal. About first question, do you mean that Pega Robotics Studio, by itself, can't create web applications unless interrogating other existing web applications?
That is correct. Generally, you wouldn't design a web application with Robotics Studio. It is focused on automation. You would be automating an existing web application. You also have the capability of editing existing web applications in real-time as well (for example, adding a button to a webpage after it is loaded which starts an automation or some other action).