Hello, .

UPSC Civil Service Exam consists of three stages-

Preliminary : First Stage is MCQs based exam, it has two papers and each one consists of 200 marks,its numbers ain't added to final list, you just need to score the required cut offs against your category.

First is GS Paper-I which consists of the following topic such as History of India and Indian national movement, Indian and World geography, Economic and social development- sustainable development, poverty,inclusion demographics, social sector initiatives etc,Indian polity and governance-constitution,political system,Current Events nationally and Internationally, General Science,General Issues on environmental ecology,biodiversity and climate change. you just need to score the required cut offs against your category.

Second Paper is CSAT which is qualifying by nature,you need to score merely 33% which is 66 marks out of 200, it includes following topics such as basic numeracy, comprehension, logical reasoning and analytical ability, general mental ability, decision making and problem solving, interpersonal skills including communication skills.

Mains : This is a second stage which it's more of a subjective type, you are required to write answers for Four General Studies Paper such as

GS-I includes Indian Heritage and Culture,History and Geography of the World and Society,
GS-II includes Governance, Constitution, Polity, Social Justice and International Relations,
GS-III includes Technology, Economic Development, Bio-Diversity, Environment, Security and Disaster Management,
GS-IV includes Ethics, Integrity, and Aptitude
Two Optionals,
One Essay,
Two Language Paper

Hence, to answer your question apart from your optional subjects you are also required to read the syllabus of Prelims and Mains, as aforementioned which includes geography, economy, ethics ,history, science and tech etc.