[TheMajor]

All those characters can be typed without any extensions. When using the US-International keyboard setting in Windows, some characters can be typed using dead keys, and others using AltGr (right Alt key, or the left CTRL+ALT).

Some examples

Using dead keys:

<press ", then press e afterwards> outputs ë
<' & u> outputs ú
<^ & i> outputs î

Using AltGr (right Alt key, or CTRL+ALT):

<AltGr + s> outputs ß
<AltGr + 5> outputs €
<AltGr + n> outputs ñ
<AltGr + z> outputs æ

You can always use SHIFT:

<Press ", then press SHIFT + A> output: Ä
<SHIFT + AltGr + z> output: Æ

Much easier and faster than typing Alt+decimal character code.