Please refer to the following link on how to customize it. https://codingnconcepts.com/spring-boot/change-default-feign-client-implementation/

<dependency>
  <groupId>io.github.openfeign</groupId>
  <artifactId>feign-httpclient</artifactId>
</dependency>

IN application.properties or yaml file 

feign.httpclient.enabled: true


@Configuration
public class FeignClientConfig {

    @Bean
    public CloseableHttpClient feignClient() {
PoolingHttpClientConnectionManager connectionManager = new PoolingHttpClientConnectionManager(10, TimeUnit.MINUTES);
    connectionManager.setMaxTotal(1000);
    connectionManager.setDefaultMaxPerRoute(20);
     CloseableHttpClient httpClient = HttpClients.custom()
                                                     .setConnectionManager(connectionManager)
                                                     .build()) 
 
     Thread th = new IdleConnectionMonitorThread(connectionManager);
 
     th.start();
    return httpClient;
    }
}

  static class IdleConnectionMonitorThread extends Thread {
 
        private volatile boolean shutdown;
        private PoolingHttpClientConnectionManager connMgr;
 
        public IdleConnectionMonitorThread(PoolingHttpClientConnectionManager connMgr) {
            super();
            this.setName("TCP-Ideal-Monitroing-Thread");
            this.connMgr = connMgr;
        }
 
        @Override
        public void run() {
            try {
                while (!shutdown) {
                        sleep(TimeUnit.MINUTES.toMillis(1));
                        // Close expired connections
                        connMgr.closeExpiredConnections();
                        // Optionally, close connections
                        // that have been idle longer than 60 sec
                        connMgr.closeIdleConnections(10, TimeUnit.MINUTES);
                }
            } catch (InterruptedException ex) {
                // terminate
            }
        }
 
        public void shutdown() {
            shutdown = true;
        }
 
    }

It's impossible to tell without seeing your Spring setup, especially if your application is complex like you said.

Since you are using Spring annotations to declare your @FeignClient you most likely are depending on spring-cloud-netflix-core. If you are enabling defaults with @EnableFeignClients take a look at FeignAutoConfiguration class. This class registers the actual HTTP client bean. If you add both feign.httpclient.enabled and feign.okhttp.enabled properties (which IMO is a weird setup) try debugging FeignAutoConfiguration to see which Client feignClient() bean will be registered in the Spring context.

Alternatively enable wire logging in all HTTP client libraries and see which one actually executes the request based on logs.

In the bean that we are injecting we have to provide feign.Client's implementation. Simplest is new Client.Default(SSLSocketFactory, HostnameVerifier). I changed the httpClient @Bean injection in code in my question from:

@Bean
    public CloseableHttpClient client() {
 CloseableHttpClient httpClient=null;
         try {
                    //... Skipping code for brevity.  
                    //here creating "sslSocketFactory" used in the HttpClient builder below
                    httpClient = HttpClients.custom().setSSLSocketFactory(sslSocketFactory)
                    .setMaxConnTotal(10)
                    .setMaxConnPerRoute(10)
                    .build();
                
            }catch(IOException | KeyManagementException | UnrecoverableKeyException | NoSuchAlgorithmException | KeyStoreException | CertificateException e) {
                System.err.println("Exception during creation of HttpClient. : "+e.getMessage());
            }
        return httpClient;
    }
              

to :

@Bean
    public feign.Client client() {
      
       feign.Client client=null;
        try {

      //... Skipping code for brevity.  
                    //here creating "sslSocketFactory" used in the HttpClient builder below
      client = new Client.Default(sslSocketFactory, new DefaultHostnameVerifier());
            
        }catch(IOException | KeyManagementException | UnrecoverableKeyException | NoSuchAlgorithmException | KeyStoreException | CertificateException e) {
            System.err.println("Exception during creation of HttpClient. : "+e.getMessage());
        }
}

Learnings from the experience:

When the documentation says, "You can customize the HTTP client used by providing a bean of either ClosableHttpClient when using Apache or OkHttpClient when using OK HTTP."

And then people say they provided CloseableHttpClient as it is in this question which is not answered properly. The bean injection in this will never work.

On the top of that, the OpenFeign's Github documentation talks about using ApacheHttpClient.

This may leave a person confused. The second part in my question "Other part which I didn't understand, how the Spring was going to hook this custom CloseableHttpClient to Feign as it claims..... the 'client' field in this class is of type feign.Client "

Answer:

There is no magic, what the ApacheHttpClient OpenFeign Github documentation talks about is OpenFeign's wrapper on the Apache's HttpClient library ApacheHttpClient it implements feign.Client interface.

And this ApacheHttpClient implementation is not available in feign core 10.1.1 dependency that comes with Spring Boot starter for openfeign.

1. Is open feign a web client, or is it a library which uses a web client?

Feign is a REST Client and it use a httpClient.

2. How do we find out what web client its using?
3. how would we change it?

Look at you application.yaml

Search for something like: feign.*client.enable=true

A second way to define the client is with Annotation:

@ConditionalOnProperty(value = "feign.httpclient.enabled", matchIfMissing = true) // enables Apache httpClient

But most likely you didn't define it. Than it's the default:

java.net.HttpURLConnection