that's an invalid config. It should look like that:

<framework:cache name="app" adapter="cache.adapter.filesystem" />
<framework:cache name="system" adapter="cache.adapter.system" />

Prototyped array nodes work in XML by repeating the tag, and the key is hold by an attribute (as XML does not have any way to represent a map, as attribute names and tag names cannot be dynamic in XSD). Btw, that's the reason why the method of the Config\Definition system to mark a prototyped array node as a map is named useAttributeAsKey and accept an argument: it relates to the fact that XML needs an attribute to store the key.

Note that I haven't checked the actual config to be sure that the attribute holding the key is name in that case. So my config might be wrong. But name is the most used in the Symfony configurations so that was an easy guess.

Hm.. I think you are considering the pools key.

Here is some Yaml that I would like to convert to XML.

framework:
    cache:
        app: cache.adapter.filesystem
        system: cache.adapter.system

        pools:
            acme:
                adapter: cache.app
            redis:
                adapter: cache.adapter.redis

Also note, what I currently have written in this PR is copied from the docs: https://symfony.com/doc/current/cache.html#configuring-cache-with-frameworkbundle

hmm, looks like the XSD has been created putting those a child elements rather than attributes (which is not consistent with other parts of Symfony): https://github.com/symfony/symfony/blob/8361713367e2735fd9b63cd2b6661411a0339811/src/Symfony/Bundle/FrameworkBundle/Resources/config/schema/symfony-1.0.xsd#L275-L286

See symfony/symfony#46594.