Difference between \footnote and the pair \footnotemark and \footnotetext

I would say it is bad practice to use it as you show in a paragraph, certainly with white space before it.

As shown the footnote mark will appear where the mark is placed but the footnote text will be placed at the bottom of the page that ends the paragraph which may well be the next, or even later, page.

In cases where you have to split them this way it is usually because the text is in a box, so you can use \footnotetext immediately after the box and then it will again appear on the same page as the boxed text and so the same page as the mark.

Use https://www.ctan.org/pkg/sepfootnotes to manage the footnote content (you can even use another file to store your notes):

\documentclass{article}
\usepackage[paperheight=3cm]{geometry} % (not needed) just for the example

\usepackage{sepfootnotes}
\sepfootnotecontent{1st}{1st footnote}
\sepfootnotecontent{2nd}{2nd footnote}

\begin{document}
Some text\sepfootnote{1st} and more text\sepfootnote{2nd}
\end{document}