i wanna say i genuinely appreciate you asking politely. not everyone does that, so im happy to answer in detail.
for manual chairs the sitting part by itself might be somewhat similar to sitting in a car, though there are key differences, mainly the length of time being a relatively short car ride versus using a wheelchair up to 24/7. you also have to consider that wheelchair users are very very likely to have conditions that affect the ability to sit upright comfortably for an extended period of time. some people, such as those with SCIs, have poor trunk control (trunk sort of means abdomen) which means they may slump down in their seat (there are ways to mitigate this via the way ones chair is set up but i digress). people with hip problems can develop pain or dislocations due to the position ones hips must be in, as a few examples. for me personally i get bad soreness and fatigue in my legs from having them bent at that angle, i can only truly be comfortable with my legs fully elevated. and even with my $600+ cushion it starts to hurt after a while.
this is even more of an issue for power chair users, especially those with group 2 or 3 chairs. pressure sores like i mentioned are exceedindly common, especially for people who are very likely not be able to shift positioning independently, which is why things like tilt and recline exist, to take pressure off those spots (among other things like decompressioning the spine). someone who uses a massive bespoke powerchair may still experience extreme pain or other symptoms while sitting in their chair, and the only place they can actually be somewhat comfortable is in a special bed. its not relaxing to get to sit in a cushioned powerchair all day every day, because the people that need them often cant tolerate sitting for more than a short period of time.
and we havent even started talking about actually moving in a wheelchair. we cant talk about manual chair use without talking about repetitive stress injuries. your arms are not limbs that are designed for ambulation, they don't have nearly as much muscle as your legs and as such it is much harder to use your arms to get your body *and* often very heavy medical equipment around. is not an easy task even on completely smooth, flat, even ground, and given that the majority of ground that is traversed by the average person is neither smooth, flat, nor even, it's all that much more difficult. and again you have to consider that wheelchair users are disabled people who, more likely than not, have some sort of body systemic disorders that affect more than just the legs, which can make pushing all the more difficult. someone with me/cfs might push themselves into a pem flare. i have eds which is a large part of why i need my wheelchair, but it also affects my shoulders which means i get a *lot* of shoulder pain. ive got torn tendons in my shoulders and ive only been using my chair for 2 years and change, with a power assist. there are some people who desperately need a wheelchair but genuinely cant use their arms, shoulders to self propel without extreme pain, and powerchairs are prohibitively expensive, so lots of people just go without even though walking is extremely difficult.
and now we must talk about the difficulties of navigating the world around us *on top of* the difficulties of using mobility aids in the first place. imagine being completely barred from like half of the places you want to go to, and half the places you can actually go are like at least 4x as hard to navigate due to narrow aisles/paths, blocked aisles/paths, people that seem to lose all sense of spatial awareness when a wheelchair user enters their vicinity, cracks and bumps and other hazards, people leaving objects in the middle of the path, "accessible" entrances/parking spots being very out of the way, lack of automatic doors, lack of accessible (CLEAN) bathrooms, i could keep going.
theres also the fact that human bodies receive a lot of information about their surroundings via proprioception, which gets very messed up when you're in a wheelchair. you no longer have access to the movements that your body does to understand the space its in, which makes navigating the world even more difficult because we don't have that sensory feedback. and for a population who is way more likely to deal with brain fog and other cognitive impairments you can see why this takes a lot of mental energy on top of physical.
there's even more that i could talk about but this post is already quite long. here's a link to a reblog my friend made that has some sources attached to it about repetitive stress injuries and other complications that might also be helpful.
as a conclusion, wheelchair usage requires a lot of effort, physical and mental. it is usually better than not being able to go anywhere at all, but not always. so keeping all of that in mind, try to share some extra kindness to the wheelchair users in your community.
thanks again for asking politely!