Under old fashioned thinking, a doorway page is simply one that is optimized to rank high in search engines. Once visitors reach your doorway page, they will see a brief description of what your site offers. Naturally, your doorway page must be carefully written so that it will glue your visitors' eyeballs immediately and "compel" them to click the links that lead to your main website. But remember: getting good traffic is ONLY part of the game. You also need a super-charged website that is designed to "sell" effortlessly to your new-found visitors.
Now that was the way they were promoted in times past. These days, the reality is different. Automatically generated pages containing repeated words and phrases, often surrounded by gibberish are not a practical way to promote your web site any more. Likely, any such pages will be treated as spam by the search engines. If you don't mind that, then live with it. In time you will find your site rankings drop back until your efforts attract only those who are likely to make the same mistake. Such pages are destined to be relegated to the status of proverbial lepers.
If you make something unique, (say, a car driven by compressed air for instance) and publish the details on a web page, who else is going to have one like that? Uniqueness will result in good search engine ranking. Think about that for a minute. Once submitted to the search engines, it occupies a unique category, possibly with only one member. It will always rank highly for anyone searching for that term. To test this, I did a search for the term ""compressed+air+car"" using the google search engine. It rendered 675 results, most of which were highly relevant".
Question your own approach to site promotion. Do you do what the others do? If so, then shouldn't you expect the results others get? Copying others is often an unwise decision. Their efforts are already on line aren't they? Did you find them with a search engine? You did! Then they already have high search engine ranking for the terms you were thinking about. Can you better it?
Educate yourself! Contrast that earlier search with a search for a common term such as "how to make money online". In this case, Google found 482000 entries with that exact phrase. Not much chance of being found there! Think carefully about what you can do that is special to you. The answer is in this page.