What is the difference between 主菜 and 主食 in terms of food items?
This is a newby answer.
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I am currently trying https://www.yellowbridge.com/chinese/dictionary.php Their answer to your question is:
主菜 = zhǔcài main course and
主食 = zhǔshí main food; staple (rice and noodles).
This is slightly different to zyy's answer in that there is no mention of a banquet. I suspect the choice between 主菜 and 主食 is not dependant on the number of people attending the gathering to share a meal or dinner, but maybe the importance of the meal or dinner? Such as the annual gathering of life-long friends to share a favorite delicacy compared to a simple family meal with one main dish at the end of a long hard days work?