Resizing and moving windows on a Mac isn't impossible, or even hard, but I spend more time trying to precisely position my mouse in the corners than I'd like. Swift Shift is a free, open source application that makes it easier by allowing you to move or resize a window from anywhere, just by holding a key and moving your mouse.
You can download free from Github (and optionally pay what you want for it if you want to support the developers). After instillation, it will live in the menu bar, but you'll need to set your keyboard shortcuts, and optionally decide whether to require mouse clicks, before you can start using the application.
You can choose between any single or combination of modifier keys for your shortcut, including Command, Control, Option, and Shift. Simply hold down your chosen modifier and move your mouse over a window and it will either move or resize. By default you don't even have to click the mouse, though you can change this in the settings; this will allow you to do things like press a single keyboard shortcut and use the left and right mouse button for moving and resizing. It's all very flexible.
By default, resizing happens from the bottom-right corner, regardless of where your mouse is—or, check the Use quadrants option and the window will resize based on what corner your cursor is in. I find this a lot easier to manage, personally, so I'm glad the option is there. You can also decide whether the window you're moving or resizing should become the active window. It's nice to see such a simple application build in such granular controls.
There are more complex Mac window management applications out there—Loop gives you a lot more control, including keyboard shortcuts, and macOS Sequoia offers basic window snapping. But Swift stands out for how fluid using it feels, and how free-flowing it allows you to be with your window arrangement. Check it out if other options aren't quite working for you.