I mean, the entire forum can - rightfully - be considered a SEO course since it's full of threads on everything you can think of about SEO. So, if you have time and motivation you can look up any topic you want / need to know about with regards to SEO and you'll find 100s of threads answering your question, even on personal level since many people have had the same questions as you.
Or, if you absolutely need SEO courses... or, rather course-style SEO tutorials, I can link you to one of the posts that I've made last month and which contains links to many tutorials. But again, most of those links are just threads that I've copy-pasted from this forum, so you can find them yourself with just a bit of search (or by going through each sub-forum and reading the pinned threads, you can learn a lot that way too).
Regarding investing in established sites, what I can tell you from past experience (I did sell a couple of websites on Flippa over a decade ago, and I've noticed all sorts of weird questions that potential buyers have asked me and which - when you think about it - they make sense, so I remembered those things for future reference), is that if you want to buy profitable websites you must first inquire the seller about a few things, such as...
1. first and foremost, proof of revenue. You want sites that have had a constant, or growing revenue trend in the last 6-12 months, not a declining one.
This is the utmost important thing when you want to buy websites that make money, and if you see that the revenue has been constant or growing steadily over the last 6-12 months then this is a sign of a GREAT website and the most important metric you'll ever need.
If the revenue has been going down over the last 6 months, it might still be a great website, but you'll have to look into why the revenue has been going down. It could be loss of rankings (if the website generated traffic and sales via google, for example), or it might have been something else (poor CTR, or technical issues with the site, or whatever). So, you need to discuss this with the seller and politely ask him / her to provide you proof for why the revenue has been going down because there's a chance that as soon as you buy the website the revenue stops coming in completely, at which point you will have just invested in a dud.
2. secondly (and if the revenue is OK)... well... everything else is irrelevant at this point, but just to make sure that you're buying a healthy website check its standing with google! Check:
- how many pages it has indexed
- how many keywords it ranks for
- how many pages rank
There are probably more things you could check, but these 3 are the most important ones and - just like in the case of revenue - you will want to see all of these metrics growing (best option) or at least being stable over the last year or so.
If the number of indexed pages have been constantly dropping from google then there's something that google doesn't like about this website, and this means that in a few months after you buy the website it's possible that you lose most / all google traffic and, as a consequence, you might lose the revenue, too. So, make sure to check the behavior of indexed pages over the last 6-12 months.
And likewise the number of keywords and ranked pages, actually. A slow decrease in any of these means that the site is losing rankings / traffic, which means that it loses revenue.... although, it could be possible that the ranked / indexed pages drop but the revenue stays constant, so checking only the SEO metrics might not tell the full story if the website has other sources of traffic that converts into sales, such as... I don't know... social media maybe, or reddit, or local / national directories, or whatever...
3. something else you need to ask proof for, from the seller is that he's not generated all of that traffic and revenue via PPC. If they make money with PPC (Google Ads, Facebook Ads, etc) then - as soon as you buy the website - all of the traffic and money will stop, obviously. Unless, you resume the profitable campaigns, and many buyers don't want to have anything to do with this. So, make sure that you're not buying a PPC-based business, or - if you are willingly and knowingly doing this with the intention of resuming this business model once you acquire the website - demand (politely, of course) from the seller that they transfer ALL of the profitable assets (ads, keywords, etc) over to you so you can resume the profitability.
4. proof that the website is not penalized by google... although, if the SEO metrics that I've explained at point #2 are OK, this usually means that the website is not penalized by google.
But just to be safe, ask for proof from the seller, any proof they can provide (GSC snapshots usually) is great. Also, type site:website.com into google and see how many results show up (replace website.com with the actual website you wish to buy, obviously!)
If you only see 4 results, or 7 results, or 9 results.... basically, under 10 results, but 4 or 7 usually and you know for a fact that the website has 100s or 1000s of indexed pages and keywords that it ranks for (because you have checked this in point #2) then something's not right somewhere. Unfortunately, I can't say what exactly is not right because it could be a plethora of things that could be wrong, but just keep this tip in mind!
Generally, when you run the site: search you'll want to see as many results as the number of indexed pages, or as close to that number as possible. If this happens, you're good! But if the number of results returned by the site: operator is way lower than the number of indexed pages then something's wrong somewhere, and you need to run deeper analysis.
Anyway, that's all I can think of at the moment, if other people have more tips they are free to share them. But this is a good starting point for researching profitable websites that you intend to buy, so start here!