Alt codes (Alt + numbers on the keypad) do not exist. The ⌥ key (Alt on a PC keyboard) is a third layer of the keyboard instead, and the common characters on a US layout are:
| Character | Keys |
|---|---|
| — em dash | ⌥⇧- |
| – en dash | ⌥- |
| € | ⌥⇧2 |
| £ | ⌥3 |
| © ® ™ | ⌥G ⌥R ⌥2 |
| ° degree | ⌥⇧8 |
| é è ê ñ ü | ⌥E then the letter; ⌥`, ⌥I, ⌥N, ⌥U for the other accents |
| ß ø å æ | ⌥S ⌥O ⌥A ⌥' |
Anything else: ⌃⌘Space opens the Character Viewer, the Mac's Character Map with a search box; type "arrow" or "check" and double-click. The same window holds the emoji.
Holding a letter key normally pops up its accented forms with numbers to pick one. Windows users tend to prefer key repeat; with the accent popup off, the ⌥ combinations above are the way. To see your layout's third layer, System Settings → Keyboard → Input Sources → Show Input menu in menu bar, then Show Keyboard Viewer and hold ⌥.