Why the passed in parameter type is by default the value of generic type

Question

I saw a generic function like this:

const myFun = <T>(p: keyof T extends 'foo' ? never: T)=> {
    console.log(`...`);
 }

myFun('foo') // good
myFun({foo: 1}); // bad: Type 'number' is not assignable to type 'never'

I have one question:

When calling myFun, I only pass in either string or object, I don't explicitly tell typescript the value of generic type T like myFun<string>(...), it seems the function is already considering the passed in parameter type as the value for T & starts evaluating the condition keyof T extends 'foo', I wonder why is that? Is that the default behavior of typescript that the argument type is by default the value of generic T?