Suppose I wanted to discuss, say, typewriter repair. How could I find the appropriate community? How do I avoid having my questions deleted because they are “off topic”? How do I find posts/comments/answers related to my interests?

Very good! You might consider clarifying who “we” and “our” refer to on pgs 1-2. It’s possible to get the impression that Fedican is the Fediverse. Page 2 could be clearer; “what do we actually do” retitled “how does it work” could be first and much of the other text relating to Fedican placed under “3. Organization”. The detailed section on Lemmy is great.