You can display a dynamic map layer from a map service. If the service is published in a different coordinate system than the map, it will automatically reproject on the fly. More information about Map Services can be found in the L.esri.DynamicMapLayer documentation.
<html>
<head>
<meta charset="utf-8" />
<title>Display a dynamic map layer</title>
<meta name="viewport" content="initial-scale=1, maximum-scale=1, user-scalable=no" />
<link rel="stylesheet" href="https://unpkg.com/leaflet@1.9.4/dist/leaflet.css" crossorigin="" />
<script src="https://unpkg.com/leaflet@1.9.4/dist/leaflet.js" crossorigin=""></script>
<!-- Load Esri Leaflet from CDN -->
<script src="https://unpkg.com/esri-leaflet@3.0.10/dist/esri-leaflet.js"></script>
<!-- Load Esri Leaflet Vector from CDN -->
<script src="https://unpkg.com/esri-leaflet-vector@4.2.3/dist/esri-leaflet-vector.js" crossorigin=""></script>
<style>
html,
body,
#map {
padding: 0;
margin: 0;
height: 100%;
width: 100%;
font-family: Arial, Helvetica, sans-serif;
font-size: 14px;
color: #323232;
}
</style>
</head>
<body>
<div id="map"></div>
<script>
const apiKey = "YOUR_API_KEY";
const map = L.map("map").setView([37.71, -99.88], 4);
L.esri.Vector.vectorBasemapLayer("arcgis/imagery/standard", {
apikey: apiKey
}).addTo(map);
L.esri
.dynamicMapLayer({
url: "https://sampleserver6.arcgisonline.com/arcgis/rest/services/USA/MapServer",
opacity: 0.7
})
.addTo(map);
</script>
</body>
</html>