Your question suggests that you regard copyrights as property rights. This is a completely understandable misconception - big content creators push this agenda quite hard - but it is a misconception. Copyright is instead a system of time-limited monopolies on certain acts which the law grants to copyright holders for the ultimate purpose of encouraging creativity.
When Alice creates content, she doesn't own it, but she has a copyright interest in that content; she is a rightsholder therein. It is appropriate, though not a requirement, that she alert the world to this interest with a
(c) 2020 Alice-style declaration, preferably at the top of each constituent file. If she publishes it on GitHub, let's say under the MIT licence, she still has a copyright interest in every copy of that content that is made.
Generally, when you fork that repository and edit a file the result is a file in which two people now have a copyright interest - you and Alice. It is appropriate, though not a requirement, that you signal this interest to the world by inserting into the file a declaration such as
(c) 2020 Vivek Singh, preferably up by Alice's declaration. Any new file you create which contains new code which is derived (in copyright terms) from the existing code (which is likely to be most of the new files you create) should have both copyright declarations at the top. Any new file which contains only new code with no copyright relationship to the existing code should have your copyright declaration at the top.
For those files which contain both your copyright declarations, both of you are rightsholders in the file, and the consent of both of you is required to do things with that file that are controlled by copyright (including but not limited to copying it and modifying it). Alice has indicated her consent by licensing the content to the world under the MIT licence; how you licence the modified version will indicate to the world what, if any, rights you grant it. You will also need to make a clear license declaration in files with only your declaration in, to clarify the same.