I see many questions (e.g. this) have other relevant questions linked to them. How do I do it?
You don't do it yourself. The system read the tags and url in comments/questions/answers for relevance:
In this example, all questions share the ebook with your question:
The linked question is because the answer includes a link to that question:
Also, it shows the questions that are back-linked to yours. Check this out. https://askubuntu.com/questions/linked/13758?lq=1
These links are generated from actual links in the question, answer, or comments to either of these.
You can put a link on one side and the link will actually go in both directions.
As you can see, if you follow the linked question to this one you see that there is quite a number of linked questions, even though it itself doesn't contain links to them.