Reddit is owned and operated by a for profit company, and order goes something like
User -> subreddit moderator -> Reddit admin -> Reddit executives / investors
With the fediverse, there is usually an open source software that anyone can download and run on their server. That software is free, and the server costs are usually paid for through donations or grants from non profits. The order there goes:
User -> community moderator -> instance admin
The instance admin may need to follow the laws of the country they're living in, but usually there isn't a financial incentive to make decisions. That's one of the key differences.
So you could think of Lemmy as a collection of different Reddits that can all communicate with eachother. They all behave similar because they're all running the same software (Lemmy), and they can all cross communicate because they all speak the same language (ActivityPub). That's the other key difference, being decentralized. If one instance goes crazy and decides to ban third party apps, or monetize, or sell user data, it doesn't bring everything down.
if it's community specific, you can report content to the mods
if it's instance specific, like some bad user or community showing up in the feed, or wanting to replace a moderator, you can ask an instance admin
How do you know an instance or admin is trustworthy?
There's no consistent answer to this one yet. There's no centralized place to find that info. Some instances are run by non profits and organizations, which is a good sign. If they are open and transparent about their financials (ex. OpenCollective) that's a good sign. If the admin is YOU (ex. Some people self host an instance just for themselves), then you can be 100% confident since it's just yourself.
For myself, I picked lemmy.ca because I'm in Canada and the instance seems pretty open. Last I heard they were filing for their official nonprofit status in Canada so I'm pretty happy with my choice.
Lemmy is a bunch of sites that are individually similar to Reddit. They all have their own "subreddits" and admins and users, paid for by different people and hosted in different places. But since they are all speaking the same language, they can interact with each other, allowing you to subscribe, post, and comment with other instances of the site from your home instance (the one where you registered an account)