Unity has come a long way in terms of UI, from OnGUI functions, to NGUI, UGUI and soon Layout Builder.
Unity has quite a few options when it comes to UI systems.
OnGUI
OnGUI is still being used on some projects and in Custom Unity Editor Scripts.
NGUI
UGUI
Canvas
Render Modes
Screen Space - Overlay
Screen Space - Camera
World Space
Try and split your canvas up into as many individual canvas as possible, this is for performance reasons.
Courses
Paid Courses
References and Resources
Simulating UI Clicks: https://youtu.be/Q9-5obVm-_o?t=295