Answer Description

The two most common ways to inject pop-up windows into a web browsing session is by using a malicious application which will most likely be found by the anti-virus software, or by installing a plugin or extension to the browser that creates the pop-ups. Some free applications downloadable from the internet will install these plugins as a way to create ad revenue.

Wikipedia

A browser extension is a software module for customizing a web browser. Browsers typically allow users to install a variety of extensions, including user interface modifications, cookie management, ad blocking, and the custom scripting and styling of web pages.Browser plug-ins are a different type of module and no longer supported by the major browsers. One difference is that extensions are distributed as source code, while plug-ins are executables (i.e. object code). The most popular browser, Google Chrome, has over 100,000 extensions available but stopped supporting plug-ins in 2020.