how can i write unique description for hundreds of similar products?

The product page will be a specific long tail keyword term - But honestly this is the last thing you should worry about... this is the page all of the duplicate attribute content will be on. you need to focus on CONTENT that creates a flow of traffic that you then send that traffic to the product page. You are going to have lack luster results trying to get organic traffic to product pages.

Go back to the original post where you say " I saw some of my competitors did the same as the developer suggested(focus on the web content and blogs only, products can be the same) "

I think you are not understanding this concept. you also say in the original post " they quite the same except The inner size, The have the same material, same color, Same shapes "

So you want content based on "COLOR" a page like this: ( https://blog.9gem.com/meaning-gemstone-colors/ ) as an example. I would go as far as writing a piece of content to attract readers / traffic / potential buyers for EACH color seperately

Why are there different inner sizes? write a piece of content about that and send the reader to specific pages that display items with a specific inner size. Again I would write a piece of content for each size specifically

Materials are kind of like colors there are different meanings with this as well. Some materials ground people etc. Do some research and create nice informative content and then guide the reader to the different materials your bangles you are selling are made from. and again write a piece of content for each material specifically

Shapes as well... ovals and circles and triangles and squares and whatever else... there is content to be written about each shape.

When it comes right down to it THIS is where you will find success. These are the pieces of content that return the most on the time spent. THIS is what will separate YOU from your Competitors.

Hope that Helps!