ISO characters and symbols on ANSI layout/keyboard (ISO-PT example)

1) Autohotkey
This solution is intended more for the keyboards that aren’t programable (don’t support QMK for example), but nothing stops you to use it even on programable ones. I use it on DZ60 PCB.
Autohotkey consists in having a small app working in background that detects combinations of keys you click in order to trigger the output you have defined.

For example:
i have configured my compiled .ahk file to be executed every time i login into my windows account so it is completely transparent to me. My compiled .ahk file has the code below and ! means i press and hold left alt.
Combination of left alt + c results in ç
left alt + e + a → á
left alt + n + a → ã
left alt + i + a → â
The downside is that it only works on the PC that you have autohotkey configured.

!c::Send {RAlt down}{,} {RAlt up}					;ç
!e::!'									;´
!n::+~									;~
!i::+6									;^

2) QMK Firmware
If your keyboard (PCB) supports QMK or any other programable firmware variant then you can program special characters and symbols you need anywhere you want or on any layer.
With QMK you have to program a keymap.c file, compile it and upload it to your keyboard. It will work with any PC anywhere you’ll use that keyboard (under same windows settings obviously).

3) Use Windows’ Keyboard input settings: US International.

Press this key	Then press this key	Resulting character
‘(APOSTROPHE)	c, e, y, u, i, o, a	ç, é, ý, ú, í, ó, á
“(QUOTATION MARK)	e, y, u, i, o, a	ë, ÿ, ü, ï, ö, ä
`(ACCENT GRAVE)	e, u, i, o, a	è, ù, ì, ò, à
~(TILDE)	o, n, a	õ, ñ, ã
^(CARET)	e, u, i, o, a	ê, û, î, ô, â

By pressing Alt Gr you get next ISO special letters and symbols:

¡ ² ³ ¤ € ¼ ½ ¾ ‘ ’ ¥ ×
 ä å é ® þ ü ú í ó ö « »
  á ß ð           ø ¶ ´ ¬
   æ   ©     ñ µ ç   ¿

ALT Gr in combination with Shift key

¹     £               ÷
 Ä Å É   Þ Ü Ú Í Ó Ö
  Á § Ð           Ø ° ¨ ¦
   Æ   ¢     Ñ   Ç

Sources:
https://eurkey.steffen.bruentjen.eu/start.html
https://docs.microsoft.com/en-us/globalization/keyboards/kbdusx.html
https://support.microsoft.com/en-us/topic/how-to-use-the-united-states-international-keyboard-layout-in-windows-7-in-windows-vista-and-in-windows-xp-e05ce3ed-11f0-8fc2-498e-a214fa6b898f
https://en.wikipedia.org/wiki/AltGr_key