Add a AOSP dictionary

First download the dictionary file, e.g. from here. Then go to dictionaries settings, and either click on "add dictionary from file", or click on the language and the "add dictionary from file". Next select the file, and adjust the language if necessary. Alternatively you can open a .dict file in a file explorer with HeliBoard and then select the language. Note that the latter method does not work with all file explorers.

Add Custom dictionary

You can add custom dictionaries, or replace a built-in "main" dictionary. Adding is done in languages & layouts by tapping the + next to dictionaries. Alternatively you can open a .dict file in a file explorer with HeliBoard and choose the language. Each dictionary has a type, which is unique per language. So when the dictionary has type "main", it will replace any existing main dictionary for that language.

Create dictionary

Creating dictionaries is a somewhat complex process, you can find information in the readme at https://codeberg.org/Helium314/aosp-dictionaries. In the same repository you can also find more dictionaries gathered from various sources.

Emoji search

You can get addon dictionaries for emoji suggestions in the dictionaries repo. An actual search function does not exist yet.

Personal dictionary

Words added to Gboard dictionary are not suggested Gboard uses its own dictionary instead of the system's personal dictionary. See here for how to export the words.