Extremely new to all of this. If each can have the same name, then would that mean one instance of a lemmy “subreddit” that share the same name not be able to see the other?

Nope! That’s why community names are often formatted like community@website. As many instances can use the same community name as they like, everyone can see and individually interact with each of them. Even if two communities are both named tech, they are still distinct from one another by the website that’s hosting them.

No the domain name is always part of the ID unless it’s your home instance.

A very precise way to phrase this is to say:

There is no community called technology on Lemmy. There is a community called !technology@beehaw.org and a separate community called !technology@lemmy.ml. They are different communities with different mods that discuss similar topics. Their proper “names” are comprised of BOTH the topic description AND the home instance.

Every community on Reddit happens to share the same home instance, like mysub@reddit.com, but it makes very little difference if you start thinking of the sub-name as just being comprised of both parts.

Another funny wrinkle is that your home instance will often (always?) hide the instance name from local communities. So for someone with an account on lemmy.ml, !technology@lemmy.ml will look like just plain old technology. But this is just how the UI styles local communities, they still homed to the instance where your account is, and they are still most precisely and correctly described with their full identifier, including their instance name as anything else is ambiguous to people with accounts on various different instances.