As far as I know – base 64 can represent any char. ( inclduing binary)
Base64 encoding schemes are commonly used when there is a need to
encode binary(!) data that need to be stored and transferred over
media that are designed to deal with textual data
So I tried to apply it on extended ascii char ( beyond the 127)
the char :

after following the simple algorithm :
I got to :

so the value should be Fy
So why when I use online-encoder and put the value by alt+178 ,

I get this result :

What is going on here ?
Your browser sent the encoding website the UTF-8 encoding of the character. This encoding is not 178.