I'm sorry to hear about your sick parent, totally understandable that you are thinking and worrying about these things now. That being said I think you are worrying unnecessarily. You're very young and in all likelihood it's a long way in the future, who knows what will have changed in your life by then. Also surely it doesn't really matter deciding now who will deal with your clothes and other bits and pieces, is this perhaps your grief/anxiety seizing on something concrete and tangible to fixate on? Things I think you can sensibly put in place now:
-Money makes nearly everything easier so having a stable housing situation, good cushion of savings and healthy pension is something you can prioritize now.
-Take care of yourself healthwise, obviously some health problems are not avoidable but keeping fit and active, mentally agile etc and taking sensible proactive steps for a healthy lifestyle (getting good sleep, managing stress, drinking water, eating a balanced diet) may make the difference in terms of being able to stay independent for longer in old age as well as potentially improving your life now. Addressing any issues like joint problems, vision or hearing deterioration early and trying to avoid being in denial for as long as possible is sensible. If you are offered help or adaptations to your home at any stage take them even if you don't really think you need them. Similarly maybe practice at an early stage accepting help with things like cleaning before you really start to struggle, many elderly I've known really feel a lot of shame and reluctance about bringing in outsiders to help even with trivial things like gardening, housework but realistically even if you had a partner or children to help it's likely you'd need some assistance at some stage so if you can afford it why not start earlier rather than later.
-Consider living in (or making plans at retirement age to move to) a sensible, accessible property, somewhere where there are good facilities for the elderly, good public transport, amenities, things to do, sense of community, accessible healthcare etc. I can't recommend strongly enough retirement communities ('sheltered housing'), particularly 'extra care' facilities - you can move in once you are 55 usually (might be 60 by the time it's relevant to you), they're not care homes, you have your own self-contained flat and live as independently as you want to but things are simplified financially as bills and maintenance are included within your rent or service charge and things like gardening and essential maintenance work are arranged for you by the management company, there is additional care and personal assistance available on-site to be bought in if needed, often there's a restaurant or meals service available, and most importantly usually there's a good sense of community with planned activities but also lots of informal opportunities to socialise too. There are very swanky versions available with swimming pools and gyms and so on (with service charges to match!) but equally there are lots and lots run by housing trusts which really are very affordable so please don't be put off by the horror stories about the expensive versions.
-Speak to a solicitor about making a will - you can leave your money as you like (to charity or similar if you don't want it going to family) - you will need to appoint an executor who will be responsible for ensuring your estate is managed in accordance with your will. Usually the executor will arrange for your funeral in accordance with any wishes you've left (you can buy a funeral plan or just ensure there's enough money in your estate to pay) and will also ensure your house is cleared out for sale or returned to landlord if you rent. As per a PP a friend or young relative would likely be fine to do this role and it would be worth their while if you leave them some money in the will, or even simpler, nominate a solicitor as professional executor and they will take care of it all (for a fee of course). Professional house clearers will likely be brought in if you use a professional executor and will very happily deal with clothes, furniture, kitchen items etc. so don't worry about that although if there are valuable assets or even just sentimental items you'd like to leave to someone specific make sure to put that in the will.
-You can consider making a lasting power of attorney (again can be either a friend or relative or a solicitor) who will advocate for you and ensure any expressed intentions (e.g. going into a care home) are followed as far as possible should you lose capacity to make these decisions for yourself e.g. through dementia. Make sure you explain what you would want to happen in these instances to the person you nominate so they can follow your wishes.
-Personally in terms of loneliness and emotional wellbeing, I'd focus on improving my life now rather than thinking too far into the future - obviously you can't just snap your fingers and feel better overnight but e.g. if you are lonely now, what steps can you take to resolve this? Not necessarily getting a romantic partner unless that's something you particularly want, but social connections generally, through work, hobbies, friendships, volunteering etc? Even if the situation with your parent at the moment prevents taking a lot of steps forward on this right now you can make a medium term plan? The good news is that many people these days have 20 years+ in retirement in good health which is a lot of time to make new connections or reinforce old ones if you go about it in the right way so I don't think you need to assume you'll definitely be isolated in retirement (if you plan to move to shack on a hebridean island populated only by puffins with no wifi, no car and knowing no-one within a 100 mile radius it will be more challenging of course but even then there are ways and means!).