The answer to your first question is found in the help: it's for questions faced by Android users, not for questions about developing and publishing apps or ROMs. Programming questions should go on Stack Overflow instead. Problems with development tools (such as
adb), and questions about Android internals are fine on this site provided the answer is useful or interesting to end-users. For example, questions about using
adb backup to back up your device are on-topic, but questions about automating something that's only useful for testing apps are not.
The same distinction applies to hardware questions. We've some questions about using Android with certain peripherals, and about replacing broken screens or fixing other hardware problems that end users are likely to face. But questions about how to port Android to your favourite dev board, or which pins on a camera module are which, just don't fit here.
As for directing questions here, either by migration or by making a suggestion to a questioner, the answer is the same as any SE site. If you're not an active user of a site, don't migrate a question to that site. If you suspect a question might be suitable for migration, pop into chat and ask the moderators. Migrating a question to a site where it's immediately closed (or telling a person to ask on another site where it's immediately closed) just drives away new users with bureaucracy.