lemmy.world and lemmy.ml are different “instances” of Lemmy. Anyone can host one, and every Lemmy instance can talk to other Lemmy instances, and similarly any other software using ActivityPub. It’s part of the “Fediverse”, or the federated universe.
In the long term, it won’t matter unless lemmy.ml starts defederating from other instances or other instances start defederating from it. “Defederating” is a term for when the admins of an instance choose not to talk to another specific instance.
I installed Sync to try it out, and noticed the subscription model for no ads, and for basic features like push notifications and exporting a list of your subscribed communities. $2/month or $100 for a “lifetime” subscription. Immediately uninstalled it, yes it has a beautiful interface but I’ve never been fond of subscription-based apps, especially for FOSS services. A low one-time fee would be acceptable to me, like the $4 fee for Toot! for Mastodon.
The silly thing to me about the subscription is, they say it’s to “cover the monthly running costs”. What running costs do they incur if Lemmy is free and open? I’m genuinely curious, if anyone knows please tell me.