When Chinese speakers read Arabic numbers, do they still read them in Chinese? Often you see Arabic numbers among Chinese texts for example: 我想要321餃子
would that be read as "three hundred" or as "三百二十一"?

This may seem obvious but thought I'd seek clarification.  Learning the numbers has been a challenge, due to the different decimal places, so for large numbers in particular I was wondering how it works when written like e.g. 1,542,800.
I'm a native speaker. I'll read "我(wo) 想(xiang) 要(yao) 三(san) 百(bai) 二(er) 十(shi) 一(yi) 個(ge) 餃(jiao) 子(zi)".

BTW "321餃子" doesn't make sense. "321個餃子" would be correct because you need the unit "個".