Rct in brief-  all infected tissue in the tooth is removed and an inert material is filled in the hollow space.. Its better to get the tooth crowned after rct to increase its life span... Till than try having soft n non sticky food from that side..

Just get tooth capped for better results after rct as tooth becomes brittle after rct

Just go to dentist and replace the tooth cap.. And take soft food then hard foods

Do you mean post , if so then it is done to give your tooth more strength to hold the permanent filling .You can use the tooth normally with certain precautions like avoiding sticky food and biting or chewing hard food like nuts directly from that tooth and also you need maintain oral hygiene by brushing properly. And if the tooth is rct treated then I would advice to go for cap placement on it as after rct tooth becomes brittle after some time and there will chances of fracture of the tooth and the whole treatment done will be a waste