what exactly is google algorithm?

If you want something closer to the source than forum guesses, Google publishes two things that are worth reading: the Search Central documentation, and the search quality rater guidelines. The raters don't change rankings directly - their scores are used to test whether changes to the systems are improving results - but the guidelines show pretty clearly what Google is trying to reward.

It is less one algorithm than a stack of systems - spam detection, link analysis, helpful content, page experience and so on - all feeding into how results get ordered. Small changes go out constantly, and a few times a year there is a bigger core update that shuffles rankings enough for people to notice. Nobody outside Google knows the weights, so be wary of anyone selling the secret formula.