Prior to vSphere Lifecycle Manager (vLCM), the creation of an ESXi Image Profile was done separately using the vSphere Image Builder service. If you needed to download a specific ESXi Image Profile (ISO or ZIP) for importing into another environment or to simply create a bootable installer, it was intuitive to download the image within the vSphere UI.
With vLCM, both the ESXi Image and Configuration has been combined and it is now managed at the vSphere Cluster level. A couple of weeks ago, I needed to download a specific ESXi ISO from vLCM and it actually took me a bit of time to figure out where to perform this operation.
I recently came across a VMTN post where the user had the same challenge and realized I was not the only one who could not find the functionality in the vSphere UI.
If you have an existing vSphere Cluster which has been configured with vLCM using the new ESXi image management, then you can download either the ISO or ZIP by selecting the vSphere Cluster and then navigating to the Updates tab. From here, click on the three dots to expand the list of operations and click on the Export option which supports JSON, ISO or ZIP format.
If you do not have vLCM configured or if you want to download a different ESXi Image than what you have configured for a specific vSphere Cluster, then it may not be intuitive on where to download the ESXi ISO or ZIP file.
The trick that I used or rather how vLCM work is that you can create a dummy vSphere Cluster and configure the desired ESXi version and then go through the exact same workflow as describe above to download the ISO or ZIP file. You can delete the vSphere Cluster once you are done or you can leave if you think you may need to perform this operation again.
If you would like to preserve the ESXi image configuration, you can also download the JSON export which contains the configured ESXi version along with the vendor add-ons, this way you can just import the configuration without having to go through the manual selection.