Republics aren’t necessarily democratic.
A country is considered a republic when it’s government in comprised of representatives of the citizen body. But how should those representatives be selected? The concept of “republic” does not specify how. You could choose them through democratic means; You could elect a president who in turn selects the representatives; or you could have them be chosen based on who is the richest. That would still be a republic.
Democracy comes from the Greek demos, which means people or district, and kratos, which means power. Democracy is when the power is on the people.
Republic simply says the government should be ruled by representatives of the citizen body, Democracy states how those representatives should be selected.
(Please correct me if I’m wrong)
Capitalism is the economic model where the wealth created by the workers is owned by the people o whom the tools and resources required to created said wealth and not the workers who put in the work to create it.
Here is a comic that might help you understand: