How to change all requests in the Retrofit library?

Interceptor. It is added to OkHttpClient, which is used by Retrofit. An interceptor can modify the request before sending and the response before processing.

interface Interceptor {
    fun intercept(chain: Interceptor.Chain): Response
}

Create your own implementation of Interceptor and add it to OkHttpClient.Builder:

class MyChangingInterceptor : Interceptor {
    override fun intercept(chain: Interceptor.Chain): Response {
        val originalRequest = chain.request()
        val newRequest = originalRequest.newBuilder()
            // Example: add a header
            .addHeader("X-Custom-Header", "SomeValue")
            // Example: change URL
            // .url("https://new.api.com" + originalRequest.url.encodedPath)
            // Example: change method (if needed)
            // .method("POST", originalRequest.body)
            .build()
        return chain.proceed(newRequest)
    }
}

Add the interceptor to OkHttpClient:

val client = OkHttpClient.Builder()
    .addInterceptor(MyChangingInterceptor())
    // You can add multiple interceptors
    // .addInterceptor(AnotherInterceptor())
    .build()

Use the created OkHttpClient when creating a Retrofit instance:

val retrofit = Retrofit.Builder()
    .baseUrl("https://your.api.com/")
    .client(client)
    .addConverterFactory(GsonConverterFactory.create())
    .build()