SEO in 2019
If you talk about SEO then keep it in mind that it starts with content, so content is the king of any website. Content is the medium of your thoughts or expressions. Unique content is very important for optimization of your site. Plagiarism is enemy of optimization of your site. There is no place for copy/paste in case of SEO. Context is also necessary; if you touch a niche in your content then tries not being out of your context. The content and context should be a value addition to its visitors.
The other important term is comprehensive guide. The content should be in depth and easy to understand. It would be helpful to understand Google business model. Google earn money through ads. When people use Google then ads are showing on websites, so it generates revenue for Google. People trust Google because it gives accurate answer. Google gives best answer according to its algorithms.
Next term is on-page optimization. When you create a website then Google will index it. If you have structured your website in well manner, for example you use <h1> tag which mentions the main title and <h2> represents sub-title and so on, so the Google understand it very easily and called it on-page optimization.
Branding is very important for SEO. It is trust of visitors on a website, so ranking of such website has no effect on their visitors. Branding of your website is done through social media marketing like YouTube etc.
The other term in SEO is user interface and user experience. Interface is the visual view of your website, how it looks and what theme or color is used. User experience is how much time your website takes to open or how many ads pop-up when it landed. All these things are considered by Google. Bad experience will damage the traffic. Google count the time spent by a visitor on the website, called dwell time, if the dwell is high then Google will try to keep the page up. Your website should be mobile-optimize because Google prefer mobile-optimization. If user interface and user experience is good on mobile then Google will mark it up.