As far as kbin is concerned, it will appear under the “boosted” category. Some platforms handle this differently. To take a random user as an example, this fosstodon user has a bunch of posts which will show up separately from their boosts when viewed from kbin. But looking at their profile from Fosstodon itself, you will see posts and boosts mixed together as is the norm on Mastodon.
That’s the federated aspect of those platforms at work. Assuming both instances of a given platform on the Fediverse (here, Fosstodon and kbin.social) are federated together, which they are by default, content from both will be accessible from users of the instances.
As far as kbin is concerned, it will appear under the “boosted” category. Some platforms handle this differently. To take a random user as an example, this fosstodon user has a bunch of posts which will show up separately from their boosts when viewed from kbin. But looking at their profile from Fosstodon itself, you will see posts and boosts mixed together as is the norm on Mastodon.
How are they able to post from their mastadon account? Do you link your mastadon account to kbin somehow?
That’s the federated aspect of those platforms at work. Assuming both instances of a given platform on the Fediverse (here, Fosstodon and kbin.social) are federated together, which they are by default, content from both will be accessible from users of the instances.