Aside from downloading and changing PKGBUILD directly, there is an AUR helper named Paru that shows the PKGBUILD contents before using it to build the package. I’m not sure, but seems like it opens inside a terminal text editor, so you can make changes before building.
It also doesn’t prevent advertisements carried through the website’s own domain. For example, lots of video platforms send their advertisements through the same domain as the content’s domain, so if you block that domain, you’ll also block the possibility of watching any content there. That’s why you need to have ad-blocking within the browser.