With pages, if I upload static HTML, does each update to the HTML count as a build?
With pages, if I upload static HTML, does each update to the HTML count as a build?

CNAME @ Target Proxied, or delete the current Custom domain and readd it and it should be able to change it for you
_headers file with a Pages project: does it support regex? For example, would the following work?_headers if that's possible.(.*). and (ac3| , above; it just looks that way because of how the long line broke in this view.
staging, staging.your-project.pages.dev, and it'll show the latest build for the branchCNAMETarget/(.*).(ac3|avi|bmp|br|bz2|css|cue|dat|doc|docx|dts|eot|exe|flv|gif|gz|ico|img|iso|jpeg|jpg|js|json|map|mkv|mp3|mp4|mpeg|mpg|ogg|pdf|png|ppt|pptx|qt|rar|rm|svg|swf|tar|tgz|ttf|txt|wav|webp|webm|webmanifest|woff|woff2|xls|xlsx|xml|zip)
Cache-Control: public, max-age=31536000, immutable
CDN-Cache-Control: public, max-age=31536000, immutable
X-Content-Type-Options: nosniff(.*).(ac3|stagingstaging.your-project.pages.dev