How to use Netlify only for Pull Request previews

you can’t do exactly that, no. We’ll only build pull requests that target the list of branches shown in

I’d recommend using the build.ignore script as discussed in this thread, to prevent non-PR builds from firing: [Support Guide] How can I optimize my Netlify build time? - #10 by marcus

When you get that script dialed in, the build will still start, but it will exit before any substantial build time is used.

There are other workarounds for this - things like “not sending non-PR events from GitHub”, or “send through a system like Zapier that can filter which notifications it passes on to Netlify” but those are rather more complicated to configure, so I suggest trying my suggestion first.