Your first message in three steps

import paho.mqtt.client as mqtt

def on_connect(client, userdata, flags, reason_code, properties):
    print("Connected:", reason_code)
    client.subscribe("devices/+/telemetry", qos=1)
    client.publish("devices/sensor-1/telemetry", '{"temperature": 21.5}', qos=1)

def on_message(client, userdata, msg):
    print(msg.topic, msg.payload.decode())

client = mqtt.Client(mqtt.CallbackAPIVersion.VERSION2, client_id="sensor-1")
client.username_pw_set("YOUR_USERNAME", "YOUR_PASSWORD")
client.tls_set()  # TLS on port 8883
client.on_connect = on_connect
client.on_message = on_message

client.connect("broker.connectmq.com", 8883)
client.loop_forever()

import mqtt from 'mqtt';

const client = await mqtt.connectAsync('mqtts://broker.connectmq.com:8883', {
  clientId: 'service-1',
  username: 'YOUR_USERNAME',
  password: 'YOUR_PASSWORD',
});

client.on('message', (topic, payload) => {
  console.log(topic, payload.toString());
});

await client.subscribeAsync('devices/+/telemetry', { qos: 1 });
await client.publishAsync('devices/sensor-1/telemetry', JSON.stringify({ temperature: 21.5 }), { qos: 1 });

import mqtt from 'mqtt';

// Browsers connect over secure WebSockets.
const client = mqtt.connect('wss://broker.connectmq.com:8084/mqtt', {
  username: 'YOUR_USERNAME',
  password: 'YOUR_PASSWORD',
});

client.on('connect', () => client.subscribe('devices/+/telemetry'));
client.on('message', (topic, payload) => {
  document.querySelector('#log').textContent += `${topic}: ${payload}\n`;
});

#include <WiFi.h>
#include <WiFiClientSecure.h>
#include <PubSubClient.h>

WiFiClientSecure net;
PubSubClient mqtt(net);

void setup() {
  WiFi.begin("YOUR_WIFI", "YOUR_WIFI_PASSWORD");
  while (WiFi.status() != WL_CONNECTED) delay(250);

  net.setInsecure();  // for a first test; use net.setCACert(...) in production
  mqtt.setServer("broker.connectmq.com", 8883);
}

void loop() {
  if (!mqtt.connected()) {
    mqtt.connect("esp32-01", "YOUR_USERNAME", "YOUR_PASSWORD");
  }
  mqtt.loop();

  static unsigned long last = 0;
  if (millis() - last > 5000) {
    last = millis();
    mqtt.publish("devices/esp32-01/telemetry", "{\"temperature\":21.5}");
  }
}

package main

import (
	"fmt"

	mqtt "github.com/eclipse/paho.mqtt.golang"
)

func main() {
	opts := mqtt.NewClientOptions().
		AddBroker("ssl://broker.connectmq.com:8883").
		SetClientID("go-service").
		SetUsername("YOUR_USERNAME").
		SetPassword("YOUR_PASSWORD")

	client := mqtt.NewClient(opts)
	if token := client.Connect(); token.Wait() && token.Error() != nil {
		panic(token.Error())
	}

	client.Subscribe("devices/+/telemetry", 1, func(_ mqtt.Client, m mqtt.Message) {
		fmt.Println(m.Topic(), string(m.Payload()))
	})
	client.Publish("devices/sensor-1/telemetry", 1, false, `{"temperature":21.5}`).Wait()

	select {}
}

# Subscribe (terminal 1)
mosquitto_sub -h broker.connectmq.com -p 8883 --tls-use-os-certs \
  -u YOUR_USERNAME -P YOUR_PASSWORD -t 'devices/#' -v

# Publish (terminal 2)
mosquitto_pub -h broker.connectmq.com -p 8883 --tls-use-os-certs \
  -u YOUR_USERNAME -P YOUR_PASSWORD \
  -t devices/sensor-1/telemetry -m '{"temperature":21.5}'