I know they are the plural forms of he, she, it.
When I use they as the plural form for it, it can be uncountable and countable. Am I right?
I put the rice in a bowl. It is very hot.
Be aware that the above is ambiguous as it's not clear whether "It" refers to the rice or the bowl. To be clear:
I put very hot rice in a bowl.
I put the rice in a bowl and now the bowl's very hot.
I put the rice in a bowl. Be careful, the rice is very hot.
"Rice" is uncountable when referring to a portion, and takes "it". If you want to make it countable, you would use "grains of rice". However, that would be unnatural when talking about serving food. We don't count individual grains of rice in our food.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.