You can only interact with mastodonians if they interact with something on the threadiverse first. If they leave a comment on a lemmy post, you can reply to them, but due to the way they expect federation to work, they will not announce it to the instance hosting the comm you’re in, so if you’re from a different instance than the comm, only people in your own instance will see any replies to your comment coming from mastodon.
You can only interact with mastodonians if they interact with something on the threadiverse first. If they leave a comment on a lemmy post, you can reply to them, but due to the way they expect federation to work, they will not announce it to the instance hosting the comm you’re in, so if you’re from a different instance than the comm, only people in your own instance will see any replies to your comment coming from mastodon.