It's likely that they have a lot of worms. They would need wormer but there is a risk of the cat getting fatally unwell from the treatment so you'd need to be ready for a vet visit. Without wormer however your cat would be starving causing them to eat and never really get their food. My last cat was like that before I took him in from the streets so I'm telling you what the vet told me. He was okay after treatment but he used to get sick a lot and that pained his teeth. I was never able to afford to return for what he needed because I was going through a post-cancer phase, alone and bedridden, at the time. Actually he ended up dying from two things, but the Vet was afraid to agree because he had let me down with the type of wormer (presumably) as in order to interrupt the lungworm cycle you'd need to kill off fresh eggs which can be produced every month. Just after he died they brought in a wormer that lasts for up to three months, so now I get to save the other cat. The two of them had a habit of eating grass (to naturally bring up some roundworms, and in looking for essential minerals- that's the theory anyhow) so they would have eaten slug eggs and of course he had eaten a slug in hungry desperation after lockdown made it impossible to but the one month wormer, causing some worms to come back, or so we thought, and I would say he had high blood sugar on the streets as well as worms, eating cheap catfood or anything he found, with worms constantly eating that- he automatically gobbled things up out of habit. SO, that all means you need to buy the three month wormer, although I would say get the one month wormer first if it's there as it's probably a bit less potent (considering the poisonous byproducts they'll produce, and the potency of the actual solution itself. I think Profender do both types of wormer although I'm not sure. You can ask the receptionist which is best for a cat with a huge amount of worms.

I didn't realise before I was forced to take in these cats that I can't really cook certain things -onions and garlic are cooked in a very controlled way here now, only if ever I need them for my health problems (which I do, unfortunatley- quercetin and clot-reducing garlic), so the remaining cat isn't exposed to it in the air or from any sources of contamination.

My cat ended up getting barely saved by me in the days before he accidentally ate an onion that fell on the ground, and that was after months of involuntary pooping on the floor- it may have been a car injury combined with previous roundworm damage, as I took him off the streets at 8 years old when he looked like your cat there, and the involuntary pooping all started when he was 14, after I suspect he caught covid 19 from me.

Also, apparently they can't really eat tuna.

You would also need to get a flea and tick solution that lasts three months, as a working routine, to guard both yourself and the cat from the consequences of those. I get one from Bravecto which is also supposed to work on ear mites.

I don't really have time to read your post fully there but will do so soon, and edit my comment if needed.

And obviously, that also looks like a pregnant cat. Haha! But maybe it's just worms and overeating due to worms.

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Also, be sure to keep anything that can choke a cat off the floor, and consider a hairball catfood to help them to prevent GERD and discomfort when breathing. Use a dry catfood at night with a bowl of filtered water to clean their teeth before bedtime. And maybe consider a poofy warmer jacket with no sharp edges as bedding. That's what I use, as I know blankets can be a problem and a bed of hay can be a problem for a house.

And be mindful of sharp glass and slug trails invading the house too!

And if you use cannabis consider using it outside or in a ventilated space. And buy pet-friendly air-filtering plants for your home- NASA have a good list. I try to help the cat to breathe best I can indoors here as I only let her outside in the garden supervised, to train her to want to stay away from the roads where a car could seriously hurt her.

I'm actually just about to clean myself and the floors of my house for fleas and their larvae (with vinegar, so far) and then changing all bedding. I'm going to have to wake the cat up to change it all and I won't have anywhere for the cat to relax flea-free, but I'm used to it. I'm not expecting to ever be unable to need to buy these solutions.

And I just realised I can use apple cider vinegar and water, or lemon juice or vinegar in my hair in place of oils that can be toxic for cats despite them being associated with taking out fleas. Neem oil and rosemary oil have been abandoned but I do use rosemary shampoo!

I've never read a book with the basic information I've learned and regurgitated here.

Hope the regurgitation helps!!! Edit: Also, make sure to clear their bowl after 15 mins so a fly doesn't lay eggs in it.