A couple things. From my experience with Lemmy, you can subscribe to communities you want to see, the same way you could subscribe to subreddits. There's a subscribed feed, a local feed, and an all feed.
The way Reddit handled this is that there was a default set of subreddits that everyone would get. Things like /r/pics ... Whether you were browsing as a guest or as a user, by default, you could see that sub. I believe there was an option for "all" but nobody used it AFAIK. So you started with a small default (whatever Reddit thought you should see), and went from there. I'm sure, in more recent times on Reddit, it will also show you things that the algorithm wants you to see, either because Reddit is being paid to show it to you, or because it's adjacent to your currently subscribed subreddits.
Lemmy isn't substantially different when it comes to the subscribed feed, with one big exception: you don't really start with anything. So the subscribed feed is pretty bare, but the local feed is full of anything on the same instance as you are, and the all feed is everything that's local or has been brought in by federation. There may be some limits on this, for example, to NSFW stuff, but I'm not certain and it's likely up to the discretion of each Lemmy instance admin to make those choices.
The difference is in an exclusionary mindset vs an inclusionary mindset. Reddit follows an exclusionary mindset, eg. We're only going to show you what you say you want and exclude all others. Lemmy is more inclusionary, where you will see everything unless you say otherwise.
The same functionality exists here, like it did on Reddit, to only see what you're subscribed to, but you have to go and find what you want, subscribe, and then stick to your subscribed feed.
I've personally spent a lot of time on the /c/all feed specifically to find what communities I want to subscribe to so eventually, I can just stick to the subscribed feed. I'm not too the point where I think the subscribed feed has quite enough communities to keep me engaged, but I'm getting there.
The option exists and you don't need to block entire communities to get there, but you can use block for it if you want. There's nothing wrong with either methodology.