It varies from one Fedi server type to another: some are very tightly integrated to the point where you can't tell whether someone is on one or the other. For example Friendica and Mastodon allow posting and following in both directions, so you can have a discussion between Masto and Friendica accounts where there are no visible differences.
Others are sort of one-directional, for example Mastodon accounts can take part in PeerTube comments threads and follow PeerTube accounts, but PeerTube accounts cannot take part in Mastodon discussion threads or follow Mastodon accounts. (And of course PeerTube accounts can post peer-to-peer videos while Mastodon accounts cannot.)
The best person to discuss WriteFreely's level of integration is the developer @matt