sry I just want to know would and could what is the difference in both words. why we r not using would in place of could or could in place of would

Would expresses the idea of probability and of willingness. Could used to politely ask for permission to do something.

Could is use for past capacity and politely permission. I could dance when i was 12. Could you please give me that book. Would use for strongly desire. I would go to Delhi tomorrow. Would you like to have a cup of tea?

would is shows possibilities and could is shows abilities.