If you are a teacher, you cannot get access to grades from any courses you do not manage unless those teachers give you access. This would prevent you from seeing a student's grades from other classes.
As a parent, I'm not sure if there is an option for you to view multiple grades for a single student, or multiple students on a single page.
As for organizing the gradebook, a few things are options as a teacher. If you go to the gradebook, you can sort by student at the top where it says "Student Name." Hover and you will see a 3-dot menu. That menu has options for sorting via that column. Similar submenus exist for each column of the gradebook. If you want to see just a single student, you can use the Search bar above the table to filter the entire table down to just a single student. Or you can click on a student's name in the table to pull up a side menu. This still allows other students to show up. That menu has another option titled Grades that pulls up a single summary page for that student.