istio somehow overriding default access logging format of envoy

Question

As is very well explained in this elaborate answer, you can customise istio's logging format in IstioOperator.

In my case, IstioOperator (when it comes to access logging configuration) looks like this

    meshConfig:
      accessLogEncoding: JSON
      accessLogFile: /dev/stdout

i.e. no accessLogFormat specified.

However, in StackDriver, when seeing my istio-proxy logs, I see some fields NOT defined in the default format., e.g. here is a corresponding istio-proxy log entry

response_code: "200"
bytes_received: "0"
upstream_cluster: "outbound|80||myscv.default.svc.cluster.local"
downstream_remote_address: "10.17.37.1:57362"
upstream_transport_failure_reason: "-"
user_agent: "Mozilla/5.0 (compatible; Cloudflare-Traffic-Manager/1.0; +https://www.cloudflare.com/traffic-manager/; pool-id: 8474894722kdjfhg49)"
duration: "2"
x_forwarded_for: "10.15.22.1"
route_name: "-"
path: "/health"
requested_server_name: "my.public.url.net"
upstream_host: "10.15.37.29:20000"
downstream_local_address: "10.17.37.16:8443"
protocol: "HTTP/1.1"
request_id: "32455-452d-9a7a-9310-71e098fb490R"
start_time: "2021-01-20T17:27:13.021Z"
authority: "my.public.url.net"
istio_policy_status: "-"
upstream_service_time: "2"
bytes_sent: "2"
upstream_local_address: "10.43.37.16:55496"
method: "GET"
response_flags: "-"

I can see for example istio_policy_status and downstream_local_address (perhaps there are more) which are not part of the default format.

Where does istio override envoy's default access logging format?