I haven't had any serious illnesses, so I can't really say what it's like. And I hope I am never in the situation of being able to describe what it feels like to have Covid-19. But I did have something once.
It started when I woke up one morning, with an agonising pain in my left elbow, plus some pain in my left shoulder. And I felt weak, and had a temperature. When we described this to our GP, he immediately called an ambulance, and I was ferried to hospital.
Apparently, the heart cannot feel pain, but the pain is referred to the left arm, because that's the nearest. So if you ever get an appalling pain in your left elbow for no apparent reason (you didn't just bash it into a wall) call an ambulance.
They got me into hospital, into bed, and did this test and that. My heart seemed to be beating fine, and they diagnosed pericarditis, an inflammation of the lining near your heart.
They kept me in hospital for a week, as a precaution, total bed rest. Not allowed a computer! But I persuaded them to allow me books, and that week, I learned Pascal from the Microsoft manuals.
After a week, they let me out, and soon after, I went back to work. But I immediately discovered that I could barely walk a dozen yards without getting tired. For weeks afterwards, there was a huge gap between what I thought I should be capable of, and what I could actually do. I remember walking to the nearby library to get books, and I had to stop and rest frequently.
Eventually, I got my full strength back
From what I've heard from friends, Covid-19 is a lot worse than that, even if you survive.