What are these recurring scabs and bumps on my cat's neck and ears?

Ouch! Those are some major hot spots in that photo. I would recommend having Lili seen by your vet so the area can be clipped and cleaned and any medications such as pain or antibiotics can be given to help it heal a bit faster. I'd also keep Lili from scratching or pawing at it as this can introduce bacteria and more inflammation which can slow healing.

If only Lili is affected and you're not noticing other issues like parasites, etc, it is possible there is an allergy underlying. She could be rubbing up against something causing the itchiness, or could have an ingredient allergy to something in her food (or a treat if the spots correspond with intermittent feeding of a treat or wet food). Other issues such as an underlying illness like an immune or metabolic issue can also cause scabbing and skin issues that affect only one cat in the household.

A vet visit is a good option at this point both to treat the current scabs and to see if a food trial, allergy meds, or additional testing such as bloodwork to rule out any underlying health issues may be beneficial in finding the cause.