How to let the search box only search the current page?

Search context stops at the list/library level. Once you reach a page, you have the browser search that can find terms on the currently loaded page as long as they are not in a collapsed section or something that would otherwise block the page search from finding them. If you are trying to deep link to certain terms on a page, you can use the built-in heading styles to create segments on your page. Each heading also serves as an anchor. If you have a large page of content, you can manually create a table of contents for the page and refer to your Heading anchor links to allow folks to skip to the section they are wanting to read.