SEO techniques are three type

White hat seo :White hat SEO that if we're working as a legal like: follow the all guidance of any search engine.

Content posting user-friendly
Fast site speed
Mobile friendly
Easy Site navigation
Proper and natural use of keywords inside the content and meta tags

Black hat seo : Black hat SEO that if we're working as a illegal it means against the all instruction of any search engine.

Doorway
Link farming
Spam blog commenting
Hidden links

Gray hat seo : Grey Hat SEO is not something between black hat and white hat as lot of people think of, but it is the practice of using technically legal methods to improve your site rankings.