How does one keep a flat, square, true edge with a hand plane?
With a lot of practice. I am not trying to be funny. It really does take practice. It is easy to read the steps. It is much harder to do the steps.
A longer hand plane e.g., #6, #7 or #8 will be easier to get a flat edge, but it is possible with shorter e.g., #5.
I love my hand planes and can easily get the edge of a board flat. Getting the edge to be square is another matter. I get close and then finish on my table saw. I know, I am cheating.
Our bodies tend to want to tilt the plane one way or another.
To overcome this, some people attach a fence to the side of the hand plane.
This will give you an idea.
I purchased this fence. I like it, but I may need to come up with a screw attachment. I find it is too easy for my stroke to exert too much side force knocking the fence off.
A hand plane can leave a very smooth surface, ready for glueing or staining.
If you only have a circular saw, I would not expect you will get a glue ready cut.
This is a picture of a board I cut with my circular saw using a straight edge for guidance. I later found I had a broken tooth, but even after I replaced the blade, I still do not get a glue ready edge. The thin circular saw blades flex too much.