On Windows:
1. Open the arrowheads file located here:
C:\Program Files\Adobe\Adobe Illustrator 2021\Support Files\Required\Resources\en_GB
('en_GB' will vary according to your installed language version)
2. Follow the instructions in the file:
a) draw your arrowhead according to the specifications (use the grayscale black swatch in the file)
b) add it to the document symbols (drag it to the symbols panel)
3. Save the file at:
C:\Program Files\Adobe\Adobe Illustrator 2021\Plug-ins
You can overwrite the old file if you want, but saving it at the Plugins location preserves the original in case you want to restore it. Illustrator will read the file in the Plugins folder instead of the original.