How are modals indexed on non marketo pages?

Does anyone know how modals are indexed on non marketo pages? Will Googlebot (and other search engine bots) be able to get through to index any gated content that has a Marketo "form" a user must fill out (email address, name, etc.).

"Modal" isn't really material here. Whether or not a form is presented in a popup, its Thank You URL will not be indexed if the URL only exists in the form descriptor (it is not in the page, it is just a redirection target once the form is successfully posted).

If a Thank You URL is linked directly from any other page, though, it can be indexed, and you can't stop someone from sharing your Thank You URL. Thus you should have "noindex, nofollow" on those pages.

Also, our Web developers want to understand how marketo specific content is housed/delivered. Is it frames?

I don't understand the question. Marketo forms do not use iframes (let alone frames) for presentation.