How to enter in the apple symbol not in a web browser?

Neil's ⇧ + ⌥ + K might work in (presumably) the US, but the exact combination is keyboard layout specific. In addition, some keyboard layouts — Thai, for example — might not even have the symbol available.

E.g. I'm using a Finnish keyboard layout and  is typed simply by ⌥ + A.

To find this and other symbol shortcuts, you can enable the keyboard & symbol menulet from Preferences → Keyboard and use the menulet to open a virtual keyboard. Pressing any modifier key, or modifier key combination, will change the keys on the virtual keyboard accordingly. For example, here is how the virtual keyboard looks with Finnish layout while option/alt is being pressed:

To find the  from Special characters menu the fastest way is to type f8ff to the search box (this is its unicode-code; the  hasn't any other name assigned).

Other way is to change the view to glyphs or to code tables^[1] from the drop-down list and browse the for the symbol manually — but this is not exactly user-friendly or fast, unless you know where to look for.

[1]: I don't know for sure if these are exact translations as my UI is also in Finnish. However, the items are (respectively) last and third-last in the list.

⇧+⌥+K will type the Apple logo.