What do they mean by "modular" sofa?

The product is made of multiple pieces that can be moved and rearranged to create new configurations.

What material makes up the seat cushions?

The seat cushions are foam filled.

Can you change the installation direction of the sofa? I like the chaise installed on the right side.

Yes, this is a modular sectional sofa that you can install them as you like.

What is the material of the milky white color sofa?

The material of the milky white color sofa is microfiber leather.

What is the minimum door width?

Minimum Door Width - Side to Side: 31"

I notice the picture shows a right hand with arm rest. Can I get a left hand one?

The hand rest can be installed on the left side or the right side. The installation guide: https://img2.homary.com/mall/file/2022/10/31/290151217e47400cbf43f328bab2e13f.pdf

Is this available with the corner piece on the right side of the arrangement?

You can install the sofa corner on the left or right according to what you like. The installation guide: https://img2.homary.com/mall/file/2022/10/31/290151217e47400cbf43f328bab2e13f.pdf