For miters you would use the miter gauge.
For bevels you tilt the saw blade to 45 degrees and use the fence. On a mitered cut the wood is placed flat on the table, the miter gauge is set to 45 degrees and away you go, like making a picture frame.
For making a cabinet you bevel the edges, so you can't see the joint. You set the blade over to 45 degrees and use the fence to guide the other edge of the material. Then you rotate the material 90 degrees on the table and bevel the remaining edge.
To find out where to make a bevel cut, you CAN mark the edge of the material using the tri-square at 45 degrees.Your initial mark is at the desired dimension on the top and you transfer the line to the edge so you can see it. You can NOT set the fence to the desired dimension and make a bevel cut because of the angle on the blade. The material thickness comes into play here also.
This step is not necessary IF you set the fence on the right side a bit extra, and make a very shallow starting cut to see where the blade is. You can also sight the cut by eye to determine where to set the fence for your first cut, but it takes a bit of experience to get good at that.
On a bevel cut, the blade is always tilted AWAY from the fence so as not to trap the work under the blade and cause a kickback. On a typical "left tilt" table saw the fence is on the right side of the blade and the cut is made and the scrap falls on the left side of the blade.