Sticky button @ edge of site

I am looking for a plugin of some sort of Sticky Button or Tab on the right or left side of your site [at the edge].

An example is here [left-side]: https://www.webgenerator.nl/webdesign.htm

and here [right-side]: https://www.fatboy.com/nl

I’ve looked for 4 hours now to find some kind of name for it, but I really cant find in anywhere.