Given a layer endpoint URL
https://gis.la-quinta.org/arcgis/rest/services/School_Districts/school_districts/MapServer?f=html
Layer URL format is fixed, it is always like this :
https://xx.xxxx./xx/arcgis/rest/services/feature-server-name/FeatureServer/layer-id
https://xx.xxxx./xx/arcgis/rest/services/map-server-name/MapServer/layer-id
layer-id is always a number from 0 to what ever.
There are 3 options,
Option 1
1.1 open ESRI's map viewer ( classic )
https://www.arcgis.com/home/webmap/viewer.html
1.2 click Modify map button
1.3. click +, select "add layer from web"
1.4. copy and past your layer URL (arcgis rest api endpoint), then add to map
Option 2
2.1 click Online Map Viewer
2.1 multiple layers on the same MapServer were all there.
Option 3
3.1 you can just add ?url=your-layer's-endpoint-url to address bar, then click enter key will to the same job.
All ESRI map viewer are for public user to use, no need to sign in arcgis online.
The only difference is after you sign in your arcgis online account, you can save your edited map to your organization's account.
Without sign in to any organization, the save button is disabled grey out, you can't save your map to anywhere.