The mobile friendly testing tool implies that iframe content may not be indexed in exactly the same way as Chrome. This section is here to test if there is any difference.

this is how the html is reported in the tools when they succesfully manage to load the iframes content:

    <!--@ inlined-iframe <iframe ...> @-->
    <DIV>content of body without meta tags. json-ld scripts are added.</DIV>
    <!--@ end-inlined-iframe @-->

Comments are added to indicate it's iframe based content, and the html is stripped down to just the content of the body tag plus any json-ld scripts.

This is only shown if a smartphone crawl is made and that the resource for the iframe content is successfully acquired. (This includes the mobile friendly testing tool and the rich snippets testing tool in smartphone mode). If the URL Inspection test uses the desktop mode it does not report the content of the iframe in the rendered html, nor include information like any structured data found. Does this mean sites that are not mobile first may be missing out?

Is the title tag in an iframe considered as part of the content?

The rendered html for an iframe does not include the title tag, implying it is ignored. Early search result tests confirm this.

Is content in an iframes head tag seen?

The head tag is not included in the rendered html. However some content like json-ld scripts are copied over to the body.

Are meta tags in an iframes body seen?

No, all meta tags seem to be removed. This includes those used for structured data.

Yes, this question was added via JavaScript and is present in the rendered html. This indicates that the iframe is rendered, and then the visible part (body) is included in the main pages content. Plus the special case of copying over json-ld scripts so that they can be processed for structured data.