What's going on....

Hi , the moneypage on my main website scored for 15 years in top 3 positions, on google page 1.
For some reasons it began to drop to mid page 2 last autumn.
Although on "pagespeed insights", my site always scores 9/10, while opponents have much less scores.
I have some 2 others minor moneypages on this site, which has no backlinks at all, they are as always top 1, page 1.
And have also some other websites with the same keyword emphasis and which contain less content, no backlinks....
They score much better than my main moneypage.
Could it be, due to a new google-algorhythm, i lay all of a sudden too much emphasis om my page?
Or is there a blackhat strategt behind? I explain...
My main opponent, has also a SEO company, creates 1000's of backlinks for years, trying to be on position 1, to beat me.
He never succeeded with it.
Could it be he now uses a blackhat manner?
If so, which technics are used for attacking a position?
Is this something i can find it myself, how, where?
(in my backlinks i saw he uses my pagelink on 2 of his fake wabsites (where he creates links to his customers).
Was there a big shift or change in google algorhythm recently?

I think Google changed the formula for calculating page authority. At least since the beginning of the year. Revisit the changes and adjust to them

I don't quite understand what you're asking but yes, rankings do fluctuate. A LOT! The fact that you've managed to rank top 3 for 15 years with a thin content website is a miracle to me, but you're probably in one of those obscure niches that nobody knows exists or google doesn't care about because it's so small that it's not worth wasting budget on controlling....

Anyway, if you've lost rankings (that's what I'm gathering from what you explained) you need to update / improve your content, or get some backlinks. Usually one of these 2...