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Promoting means raising the profile of the book. People can't buy it if they don't know it exists. Social media can be an excellent platform to this, if you can get enough people to share your posts (advertising on social media is difficult and can be expensive).
Having a website that either concentrates on your books and other other consumables or highlights your interests and also has a section for your books can also work extremely well. This is what I do on my website, and I've had (at this time) in excess of 1.8 million hits and average 5k unique visitors a month (and month after month, that number is increasing). Having people at NASA congratulate me on the science parts of the site helped. If people become interested in you, they become interested in your books.
Note that whichever route you take, you can never start too soon. It takes years to develop a large network on social media. While it is tempting (and useful) to join writers groups, they are terrible to sell to. If they repost for you, that's great, but they are unlikely to buy anything. What you really need to network with are readers groups - preferably in your genre.
If you decide to run a website too, post links to interesting articles on social media to drive traffic. Even so, you need to keep adding stuff. I post articles on space science and technology, and I write software for SF writers to get their hard-science calculations right, and provide that on the site as a free download for multiple platforms. I also built a clock that tells the time anywhere on Mars, and I provide the software as a download and instructions on how to build and configure one. I also maintain a large (probably the largest publicly available) dataset of summarised weather on Mars. I also write articles on writing.
As you can see, there is plenty to attract a visitor for any number of reasons, and the rest is similar enough to make them want to browse around and return. This is how you make visitor numbers grow. If you make a website in order to talk about your books, also consider selling them directly. It's a hassle (but a joyful one) packaging and posting a book to some remote part of the world, but you do get global scope and you can make more money from it: they have to pay for the cost of the book and the p+p, plus a little for your time and effort and a small amount on top, but in return they get a signed copy and possibly a dedication of their choice.
You need social media to help drive visitors to your website, and the website is where you can do whatever you want and write/post things of any arbitrary length. I use a wiki for my site, repurposed as a blog but still divided into sections and made transparent through a tree-view in a sidebar to encourage browsing, but you can use something simpler such as Wordpress.
In the end, as you can tell, it takes time to build an audience. I haven't published any books yet, but if I do, I'll have an instant reach from my website and my social media contacts.