how to put a footnote at the bottom of
the page and only in one page ?

Since Scrivener’s a drafting tool, where you can work on your manuscript in many varying sized chunks, it doesn’t handle page layout the way a regular word processor does–it’d be fairly meaningless in any case, since each document in the binder might be much less than a page, and since your formatting can be changed entirely when you compile the actual manuscript so that what you see when editing doesn’t necessarily have to be what you export.

Thus, when you’re working in Scrivener, footnotes are placed inline, after the text to which they refer, and there’s no “bottom of the page” for them to go to at that stage. When you compile, you can choose whether to export them as footnotes or endnotes, if your compile format supports it. So if you compile to .rtf for instance, you can choose in the compile settings under Footnotes/Endnotes to export footnotes as comments, footnotes, or endnotes. Choosing “footnotes” here will provide the expected end-of-page notes when you open your compiled RTF in a word processor that handles footnotes (e.g. Word).