Scope is all the work that needs to be done on the project to accomplish the goals in the project charter. To manage scope, you must understand requirements, determine work to meet requirements, decompose work for assignment and scheduling, and monitoring control the scope. Understanding project requirements is like looking through a telescope at the distant end goal of your project.
If you can see a lot of detailed requirements at the beginning, like having a blueprint to build a house, then your project has a predictive or waterfall lifecycle. If you cannot see a lot of detailed requirements at the beginning, and you feel like you need to move closer to the end goal to see it better, like developing the COVID-19 vaccine, then you have an adaptive or agile lifecycle. Determining work to meet requirements involves discussing the requirements with subject matter experts to identify all the specific, measurable, attainable, relevant, and time-based deliverables and sub-deliverables that must be achieved. Decomposing work for assignment and scheduling allows the project manager and team to break down the work into more manageable elements that can be assigned to the correct team members and tracked for completion.
In waterfall projects, scope is generally defined at the beginning of the project and then monitored and controlled for change all throughout the project. In agile projects, scope is defined and redefined at the beginning of short, time-boxed iterations called sprints, where scope is monitored and controlled only during each sprint.