My org wants to know which countries were represented at an event. A simple way to do this is to "group by" country. Does CiviReport have a concept of grouping by a column?
Reports for Contributions have grouping and quite a few in the 'Extended Reports' extension (which I guess most would use) as well. The 'Report Plus' extension has grouping also. So CiviCRM certainly has the concept of grouping. In addition the new 'Search Kit' can do grouping - and many other things, it's a really significant addition to CiviCRM.
The "group by" in your question is presumably the sql construct. CiviReport makes heavy use of sql to build it's results and has a 'group by' concept that is essentially an abstraction of that sql concept, though somewhat more flexible.
But of course, it's always more complicated than you think, so the "group by" thing in CiviReport isn't always available, it's only there when it's been setup by whoever writes the civireport "template".
If you are working with a report that doesn't have group by option already visible to you in the UI, it's probably not hard to add it. Look in the developer tab to provide the specific/template you're using for your report and you can probably get some more specific help.