Understanding the basic of SEO is more important than to learn On-Page and Off-Page directly.
You have to understand the followings:
How Google crawls the web (it works like an antivirus software to scan (crawl) the web)
How Google index the page (does it index in a millisecond, second, minute or hours? Find out why some websites take longer to index)
How Rankbrain and Humming Bird algorithms change the way you do On-Page SEO.
What was the major thing that Google made a huge success even there were many search engines before Google like Altavista, Lycos, and Yahoo.
Why it took 19 years to make such GOOGLE that you see now having one simple logo and a search box.
What are the 200 factors for ranking a site [Google hasn't released the factors officially, but Backlinko founder has done a great job for finding. They are not 100% accurate, but will give you better ideas.]
Why a searcher use Google, which websites does he like. Have you ever used Google? Go to myactivity.google.com, click SEARCH, and check how you used Google, and write down why you liked the site on a notebook.
Read this officially Google updated guide of SEO https://support.google.com/webmasters/answer/7451184?hl=en
Read about all major algorithms of Google. Understand the psychology what was the reason for releasing each update. For example, Panda was released to write for the users, not search engine. Keyword Stuffing was for the search engine to rank higher, but bad for users. Samjhy?
Understand the On-Page tactics in 2000, 2010, and 2018. [As per my view, it will make your concepts crystal clear. I have seen in Bloggers Funda, many people do On-Page that was common in 2010.]
Understand the backlink strategy in 2000, 2010, and 2018.
Read about Google Sandbox
Read this guide (every interlink too) in a week or month https://ahrefs.com/blog/learn-seo/
How technical SEO can affect your site's SEO.
Why UX and UI are more important in 2018.
Why you need to study about your users' behavior via Google Analytics, Google Console and Facebook Pixel Analytics. And how you can use this data to grow your blog.