I am a novice DIYer and recently purchased a Makita circular saw (7 1/4"). The manual says that I should always put my two hands over the saw and that it is not safe to hold the saw with one hand while holding a lumber or plywood with the other. But, when I check most of tutorials or videos on Youtube, I often find that the saw is held with one hand while the other hand is grabbing the object. Is holding a circular saw with one hand a unsafe practice? Or, is the manual just over-concerning?
Holding the saw with one hand is an advanced, and unsafe technique. You'll often find that folks in the construction industry (especially those on YouTube), do not follow proper safety precautions.
Holding the saw with two hands will reduce the chances of the saw kicking uncontrollably, if the saw happens to bind or hit a foreign object. It also insures that both hands are in a safe place while cutting.
The biggest mistake people make is not understanding how quickly things go wrong - they figure they will be able to get out of the way. Here's a kickback experiment done on a table saw to give you an idea of how quickly things go bad: https://www.youtube.com/watch?v=u7sRrC2Jpp4. The man doing the demo is well experienced and he almost lost his hand doing this admittedly dumb experiment.
Your safest route is to keep both hands controlling the saw, and not need any hands to hold the board. Even getting a helper to hold the board can be dangerous. In the case of cutting a big panel (plywood), one way to do this safely is to place the panel on top of some thick foam and cut through the board, into the foam. The board will be supported on both sides throughout the cut and you can safely keep your hands on the tool.