I will advise you to read Odoo16 documentation. Also, there is plenty of videos available on youtube. You can watch from there. I think the exam topic is the same as odoo 15 exam, so you can follow the following module: Website, eCommerce, Survey, Marketing, CRM, Sales, Purchases, Project, Timesheet, Accounting, Inventory, MRP, HR, Spreadsheet, and Studio. Besides these modules, I will also suggest you to know about some other modules like Helpdesk, Events, Referrals, Time Off, Recruitment, Expenses, etc.
The Odoo Functional Certification - Sample Test, from this sample test you will get a basic idea about the question.