The pi symbol isn't readily available on your iPhone's default keyboard, but there are tricks you can do to change that and have it available whenever you need.

To type Does not equal (≠) sign on keyboard in any and all text-editors, you can use any of the methods mentioned in this article.

Starting with builds 26200.5761 (Windows 11 version 25H2 in the Dev Channel) and 26120.5770 (Beta Channel), you can enter an en dash by pressing Win + Minus and an em dash by pressing Win + Shift + ...