Difference between const ref and const pointer, c++

I'd like to know what's the difference between const ref and const pointer in c++.
When I declare on something to be const ref, can I change it's VALUE?
or the const goes on the object?
Because I know that const pointer means you cannot change the POINTER but you can change the VALUE it points to.

for example:

const char& a; // ?
 const char* a; // pointer is const
 

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