I tried to do that, and now I’m just confused. The included glyph for the lower case n doesn’t actually fit the grid, so you can’t seem to replicate it. But also that grid doesn’t have enough resolution to do the tilde. Maybe I’m missing something?

Yeah, there are some sort of shenanigans going on in the editor. The premade letters use a finer grid than what the editor lets you work with.

It's most obvious with O, {, & and # which are impossible to draw with the grid that's presented to you.

Noticed that on desktop, the real grid, including the "half points" is shown and you can actually work with it. So it might just be a problem with the mobile version.

Enter a N or n into the Editing box, you'll see the two grids that make the glyph up along with a blank third grid on the bottom, add a small tilde in the top two rows. Or copy and paste the actual Ñ or ñ characters into the Editing box to create it new, and you can use it immediately with the alphabet textbox on the left.

The editor doesn’t understand ñ and you can’t make a tilde because it requires a minimum of four points of width.

Yes, and those glyphs don’t fit the grid. Try to redraw the n from the original font yourself. You can’t, because you can’t add points between the grid dots.