Do you need to quit programs?

What you're closing is the window, not the application. Some applications will treat closing the last window as a special application and also close themselves, others will remain open. I don't think there's much harm at all in keeping such an app open, but you will get small gains in terms of memory use if you close them, and it will be slightly 'neater' (e.g. your app switcher and/or dock will be less crowded).

 

It certainly won't provide any benefits leaving an app open if you're intending to fully close it. If you're looking to reopen windows or move between windows in that app later, leaving it open will help a little as it'll still be in RAM.

Personally I never click the X and just select CMD+Q. CMD+W to close a window and CMD+TAB to peek between open applications. I'd suggest getting into that habit; these keys are all pretty close together so it's pretty convenient.

 

It depends. Good applications won’t use any or very minimal processing power when they have no open windows or no window is currently visible (e.g. minimised in the Dock or obscured by another window or space). They do retain a certain amount of memory, but the system can decide to reclaim that memory by using compression or by writing parts to disk. Some applications support automatic termination and will quit silently, though it looks as though they are still open (e.g. in the Dock). Safari is one of the programs that does this.

You can examine your applications with Activity Monitor. You can check on memory pressure and average energy usage to get a better impression. In general, I find that macOS retains its responsiveness very well.