1) don't allow user to see photos (images) / videos (files). User need to be registered to see content. Guests can see only titles, descriptions.
2) but allow Search Systems (robots) to visit site and index it.
Or may be any other feature? I know only one that make social fully unavailable for guest and Search Systems is "Enable Lockdown".
I can see something I look in Instagram. I can search somebody in Google. But when I navigate to it, Instagram ask me to login.
Also find this article: https://developers.google.com/search/docs/data-types/paywalled-content?hl=en
Subscription and paywalled content
This page describes how to use schema.org JSON-LD to indicate paywalled content on your site with CreativeWork properties. This structured data helps Google differentiate paywalled content from the practice of cloaking, which violates our guidelines. Learn more about subscription and paywalled content.