If you want to block entire domain in kbin, that’s possible, but I don’t know easy way to navigate to the domain page. If you enter the adress manually you can do it. So for example if you wanted to block lemmynsfw.com, you would navigate to page https://kbin.social/d/lemmynsfw.com and block it with the button in the sidebar of the domain page. Be warned though - if you visit this page you might see the content you wanted to block, so treat this link as NSFW. Also, from what I’ve seen - microblog posts from blocked domain may still appear, but if you don’t use microblog view this should be enough to avoid seeing posts from blocked domain
Probably the magazine has to be federated first - and so it has to be searched from the instance. I’ll provide example using magazine AskKbin and assuming you’re using browser version of Lemmy. The local adress for magazine is https://kbin.social/m/AskKbin
Go to your Lemmy and press search button. Search for the magazine using the local adress of the magazine (https://kbin.social/m/AskKbin). Once searched, it should start federating and you can subscribe to it. You should be able to easily subscribe to the magazine as well - as it is going to appear as community in search results. Similiarly you would initialte federation with communities from other instances of lemmy, if they aren’t federated yet