I find most consumer first aid kits to be useless. So I buy what I need:
Gauze pads and cotton swabs for cleaning wounds and cat ears.
Soap for cleaning wounds. Seriously, soap and water are the best choice most of the time.
Rubbing alcohol for disinfecting scratches, particularly from cats, roses, or blackberries. (I have an inflammation reaction to those three).
Bag Balm for minor scratches and badly chapped hands.
Petroleum jelly for hairballs and chapped paw pads (yeah, my first aid supplies include cat stuff).
Tegaderm HP for bandaging--clear film bandage that you leave on until healed, keeps wound clean and moist and scabs soft. Water, shower, swimming pool proof. Recommended by wound care nurse after my mother had large skin tear. Magical stuff!
Menstrual pads for direct pressure on severe bleeding wounds. Sterile, non-stick, absorbs blood. Also nice for guests.
Vet Wrap (bandage that sticks to itself but not skin or fur). Er, coban I think it's called in human medicine? Good protection over another bandage, especially Tegaderm. Keeps idiots from chewing on sutures. Can be used with a splint in a dire emergency, or to temporarily fix broken tent poles or pack-frames when I was backpacking regularly.
Nitrile gloves for working on people or critters who aren't myself. Also for fabric dyeing and wood finishing.
Albuterol inhaler for my asthma.
Afrin nasal decongestant spray for nose bleeds (learned this from my uncle on blood thinners).
Cortisone cream for insect bites and bee stings.
Benadryl tablets for ant bites--I'm allergic!
Pulse-ox meter to measure blood oxygen levels. Good for asthma, covid, and non-verbal post-stroke mothers with colds.
Other thermometer that works on cats.
Steam humidifier for colds and flu.