Migration paths can only be set up by the Community Mod team, and a site is only allowed to have 4. Every site can migrate between the main site and its meta site. The other paths are harder to get set up and require demonstrating that there are many questions getting asked that are migration candidates. For the occasional migration, community members should flag for moderator attention.
Questions usually shouldn’t be migrated unless they are off-topic on this site and on-topic on the site they’re being migrated to. There’s more about migration in this post on Meta. There’s a lot of good information in that post - a relevant excerpt:
- Don't migrate for the sake of migration. We only migrate questions because they are off-topic on the original site. It is perfectly possible for a question to be on-topic on multiple sites, but that is not a reason to migrate it elsewhere, unless the OP requests migration. As a general rule, if someone asks a question here, and it's on-topic here, it should stay here.
- Be familiar with the destination site. If you are not familiar with the destination site, read through its about page and help center thoroughly and make sure the question actually belongs there. Don't just look at the name of a site and automatically assume that the question can be asked there. Each site has its own rules, and rules vary greatly across sites.
We don’t try to sort questions so they are asked in the “best” place. Presumably if they are asked here, the author would like the question answered from the perspective of the experts in this community. If the question isn’t getting any suitable answers and the author feels like another site might be able to help them more, they would raise a flag for moderator attention.
Suggesting another site that might be helpful to the question’s author in a comment is usually the best course. Migration is confusing, especially if the author of the question is new to Stack Exchange and doesn’t have an account on the target site.