Hello! I did an EPQ in 2020.
It's an AS-equivalent qualification that can be either project-based with a defined outcome and development journal or aiming to answer a specific question in the form of a 5,000 (ish) word essay.
You can generally do your EPQ about anything you like. It's often a good opportunity to do something related to a hobby or interest, or to do with a university course you're looking to apply to. This is sometimes particularly helpful if you are applying for more of a niche university course. I know people who did their projects about everything from converting a room in their house, character archetype analysis on a favourite book series, analysis of how the body breaks down different glucose types, developing a small-scale game, writing and illustrating a children's book ... etc etc.
Some colleges/sixth forms offer it, some don't. If your education provider doesn't offer it it's worth asking around/trying to build support from other students to see if it's possible for you to petition to get it implemented because it generally just requires someone willing to mark it, as it generally won't have structured teaching sessions. (aside from possibly general research and essay writing advice)
Usually you would do EPQ in either Year 12 or 13, which year just depends on the college/sixth form you're at.
You will gain some UCAS points for completing an EPQ.