The text editor we are using is called CKEditor which has an open-source core, but it's also a commercial product which offers premium features.
Prior to v0.105.0, Trilium used the following premium features as part of a signed agreement between the CKEditor and the Trilium team:
Starting with v0.105.0, those features have been completely rewritten as custom CKEditor plugins, licensed under AGPL-3.0.
There are some premium CK features that look like Trilium's built-in functionality but they are completely different: