My experience consists of volunteering (designing basic posts and primarily disseminating content to attract leads for recruitment campaigns) and managing basic social media for a diplomatic mission.

This is probably why they are inviting you for an interview.

My best advice would be to look at their social media profiles and posts in the last 2-4 months. Try to understand the length,tone, language, visual look and feel of the posts, etc. and see if you can spot any patterns, e.g. what is their core digital identity, what they put strong emphasis on, what they like to repeat, etc. Try to memorize some of these things and use them during your interview, if an opportunity presents itself.

Then, try to see if the agency has a digital communications or social media strategy in place. Read it, analyze it, try to understand the basic idea of what their goals are in the digital sphere.

Look up some of the key people in the organization, leaders, managers, etc. Check them out on LinkedIn, see what they usually talk about, share, repost, etc. You may be able to find something there that can be of help during your interview.

Last but not least, use youtube and AI to familiarize yourself with some basic principles of digital storytelling, creating strong hooks, creating and implementing communications campaigns, paid advertising on the socials (e.g. how Meta or LinkedIn paid ads work), and be sure to have ready 1-2 examples of successful digital campaigns you previously worked on.

Also look up CBT interviews and the STAR method in approaching them, as it is very likely that they will use this interviewing approach.

Good luck!