I believe the correct approach is by looking at purpose, and more importantly primary purpose, rather than try to describe the chair itself.

Eg, a chair is any object whose primary purpose is to sit a single individual

If it's meant to seat multiple people, it's a bench, a sofa, etc, but not a chair

If it's meant to be extra comfortable, it might be a single-seat sofa, but this itself a chair

If it's a rock, but positioned such that it's primary purpose is to sit an individual, such as next to a table, then it's a chair.

And most notably, primary purpose is subjective -- it might be understood as a chair, or as a doorstop, depending on your viewpoint, and what you use it for.

What about stools? I don't consider a stool to be a chair, I consider them two subtypes of seating. If someone told me to grab a chair and gestured to a stool I'd find it very slightly odd.

Fine

To sit an individual at close to, or at, a right angle in regards to their legs

A small rock versus a big rock; Approximating a squat, or standing, is out of bounds

But a stool may consistute a chair when sitting a child

Regardless, I'm mainly talking about the approach to defining it, and that definition by purpose rather than structure is more likely to find a correct definition, not that my example definition is sufficient

> a chair is any object whose primary purpose is to sit a single individual

So is a zabuton[1] a chair?

[1] https://www.dharmacrafts.com/01_zb/zabuton-mat

what if we add: "with one's feet firmly on the ground and legs bent at knee at more or less right angle?"

Highchair.

Now you have to find an accurate and all-capturing definition to all the terms you used (manmade, floor, backrest, ...)

It's a game you cannot win.

Plenty of ergonomic chairs don't have a backrest.

There are also ergo chairs that you "kneel" on instead!

This would classify a Honda Goldwing as a chair, but exclude the Balans kneeling chair.

Also applies to a Sofa

Now I'm doubting if car seats are chairs too...

What is a sofa if not shared chair?