Notes in Granite are Markdown, the same simple syntax Obsidian and many other tools use. You don't need to learn it to start: the formatting bar at the bottom of the editor does it for you.
Link to another note
Type [[ and start typing a note's name, then pick it from the list. The link looks like [[Project plan]]. Click it to open that note. If the note doesn't exist yet, clicking the link creates it.
See what links here
Open the right panel and choose Backlinks: it lists every note that links to the one you're reading. Outline shows the note's headings, and Graph shows its neighbours.
Add tags
Type # followed by a word, such as #meeting or #idea. Use the tag button next to search to see every tag in the vault and the notes that use it.
See the whole picture
Open Tools → Open graph view to see all your notes and the links between them. Search finds any word in any note.