This video will demonstrate for you many different variations of corner joints you might use including butt joints with hidden screws, splines, dowels, finger joints, box and dovetail joints, and more.

Tamar is an excellent woodworker and YouTuber who explains much of the same joinery above in more detail and offers instruction.  

This is a basic tutorial for how to craft a 90-degree half-lap joint.  These can be used in corners (as this video shows) or in the center of two cross members. (Tip: a 1/8" drill bit can serve as a shim/spacer for the width of the blade--or, this video show some tips for making the shim and other ways to measure/cut))

The results here are somewhat surprising, but they are scientifically tested.  Check it out.

Ten different ways to make the strongest joint, the mitered joint.