Very quick example showing how I would start a structure like you’ve got.
One post drawn and made into a component. A copy is placed at the right location for the other corner at the same end.
The first beam is added using the two post components as a reference for the length. I drew the large face with the Rectangle tool and because I used the posts to get the length, I only needed to type in the height of the beam. In this case I just typed ,7.5 and hit Enter. without the x value in there SketchUp takes the existing value from when the rectangle was drawn. Push/Pull to give it thickness and then make it a component.
Copy the beam component to the opposite side of the posts and then use Move copy to copy the four components to make the other three.
I save joinery, if there is any, until after the model is nearly complete. That makes editing the model easier and you can use the overlapping components as guides for drawing the joinery.
And a bit of modification which is simple enough.