In a way, yes, GlassFish is a Java framework. Although also much more.
It allows running and manage applications on a server, which provides resources to the applications. And it also allows building standalone Java applications, with the server embedded in it, in a way that you would expect from a framework.
On top of that, it provides standard Jakarta EE APIs, so your applications don't need GlassFish, you can run them on other servers too. Or you can easily migrte from other servers and frameworks to GlassFish if you like it more. And you can learn Jakarta EE APIs even before you will use GlassFish, or hire somebody who already knows it even though they never used GlassFish.
I thought that was Tomcat or was Tomcat just the servlet reference implementation, i can't remember. App servers like Glassfish were what operations people used before the concept of "devops". Developers wrote the code and admins/ops deployed the code on app servers like Glassfish. Devops was supposed to put developers in charge of the whole stack but every enterprise i've seen have a dedicated devops team that manages AWS/Azure/GCP and separate developer teams who write the code. So it's pretty much the same it's always been ironically.
> I thought that was Tomcat or was Tomcat just the servlet reference implementation
Tomcat itself has never been an official reference implementation of anything.
Tomcat implements various Jakarta EE APIs, most centrallly Servlet indeed, but also JSP (Jakarta Pages) and JSTL (Jakarta Tags), WebSockets and Jakarta Authentication.
The initial Tomcat was donated to Apache, then used back in GlassFish. GlassFish WAS the reference implementation of Servlet (among others), so indirectly Tomcat kinda was the reference implementation indeed. But just a fork of its code via GlassFish.
Nope. From the glassfish.org web page: "Eclipse GlassFish is a lightweight yet powerful open-source application server that fully implements the Jakarta EE platform."
It allows running and manage applications on a server, which provides resources to the applications. And it also allows building standalone Java applications, with the server embedded in it, in a way that you would expect from a framework.
On top of that, it provides standard Jakarta EE APIs, so your applications don't need GlassFish, you can run them on other servers too. Or you can easily migrte from other servers and frameworks to GlassFish if you like it more. And you can learn Jakarta EE APIs even before you will use GlassFish, or hire somebody who already knows it even though they never used GlassFish.