On your VMware vCenter Server (or VCSA), you can create local content libraries, but also connect to remote content libraries created and published on other VMware vCenter Server servers.
Note that VMware vCenter Server (VCSA) servers do not need to be linked (ELM mode). Although it works in this case too.

For this tutorial, we therefore created a second VMware vCenter Server with 2 VMware ESXi hosts.
For the example, the name of each of these servers is prefixed with "test-".

1. Create a published content library (accessible from outside)
2. Publish an existing content library
3. Create a subscribed content library
4. Deploy a remote OVF template using your subscribed content library
5. Create a virtual machine and install the guest OS from a remote ISO file
6. Delete the contents of a previously synced item
7. Manually sync an item
8. Delete a subscribed content library

1. Create a published content library (accessible from outside)

To create a content library published on your remote "VMware vCenter Server", click on "vSphere Client", then on: Content Libraries.

Next, click on the "+" icon.

Provide a name for this new content library and click Next.

Select "Local content library" and check the "Enable publishing" box so that its content can also be accessible from another VMware vCenter Server.

The "Optimize for syncing over HTTP" option allows you to speed up streaming over HTTP and reduce CPU usage. This option is therefore interesting if the subscribed content libraries are located on a remote VMware vCenter Server and you are not using Enhanced Linked Mode (which allows you to connect several vCenter Servers together).
However, if you enable this option, you will not be able to deploy virtual machines from this published content library.

Source : vSphere Virtual Machine Administration (page 85).

To protect external access to this content library, you can check the "Enable authentication" box and provide a password.

Select the datastore where the items you add to your content library will be stored.
In our case, the "HDD2" datastore of our 1st ESXi host linked to our 2nd VMware vCenter Server (test-vcsa.informatiweb.lan).

Note: in this case, these datastore are located on our hosts "test-esxi1" and "test-esxi2".

A summary of your published content library configuration is displayed.
Click Finish.

The created published content library appears.
In our case: SharedContentLibrary.

As you can see, this content library is published externally and is password protected (if applicable).
To access it, you will need to use the subscription link provided on this page.
To copy it, click on the "Copy link" button.

As with classic content libraries, you can add virtual machine templates in OVF format.

As well as other file types, such as ISO files for example.

You will also be able to see the list of subscriptions for this content library.

For the tutorial, we imported an OVF template : Win 10 v2004 x64.

As well as an ISO file: Windows 10 v2004 x64.iso.

2. Publish an existing content library

To publish an existing content library to make it accessible from outside, click: Actions -> Change settings.

Check the "Enable publishing" box.

The subscription link for this content library appears.

To protect access to this content library with a password, check the box "Enable user authentication for access to this content library" and provide the desired password.
Then click: OK.

Now, this content library is published externally.