What is this ad on my site, how can I deal with it, and how can I prevent re-infection?

Question

Malware

I will immediately note two points: the problem is not critical (the site is not so important); I am new to IT.

On 2021-03-23 ​​I found an advertisement on my self-written corporate website.
Advertising sometimes appears after authorization (the pattern is not clear to me).
When you click on it (any mouse button), it disappears.
Google Sheets is embedded on this page via an iframe (maybe this is important).
Having studied the files on the hosting, I did not find anything. Also, I did not find inputs to the control panel from other people's ip.
I don't know at what point it started (due to the low usage of the site).
I have no money (the site is made for the sake of getting experience (as you can see, I get experience) for free), so I created a question on the Toaster.

I do not understand how the attacker found my site (it is not indexed by search engines).
The only thing I could suggest was the embedding of advertisements by forces from above .

I want to hear from you at least suggestions what it is, because I'm interested in what happened.

Answer 1

Indeed, advertising appeared locally. I didn't go into more detail about what happened.