how can I add the new characters like “&” and “@” in Glyphs Mini2?
The easiest is to find them in one of the categories in the sidebar in font view. You can right click each line and get a popup with all the glyphs that are missing in that group. The two you are looking for are in Symbol > Other.
Or you add a new glyph by pressing the
+ button in the lover left of the font view. Then type the character into the name of that glyph cell and it will be renamed correctly.