Springs RestTemplate default connection pool

Yes, Spring RestTemplateBuilder uses Apache HttpClient for pooling (usage).
RestTemplateBuilder creates HttpComponentsClientHttpRequestFactory and uses HttpClientBuilder.

HttpClientBuilder, by default, sets pool size per route (host) to 5 and total pool size to 10 (source):

s = System.getProperty("http.maxConnections", "5"); 
int max = Integer.parseInt(s); 
poolingmgr.setDefaultMaxPerRoute(max); 
poolingmgr.setMaxTotal(2 * max); 
            

To check connection pool logging set logging level as follows:

org.apache.http.impl.conn.PoolingHttpClientConnectionManager=TRACE