Whether Biden won fair and square or not, as far as the military is concerned he IS the Commander in chief and must be obeyed, UNLESS his order is clearly unconstitutional, i.e., unlawful. He cannot order a subordinate to murder someone, for instance. Perhaps it will help if we remember that there is something called the Posse Comitatus Act that bars federal troops from participating in civilian law enforcement except when expressly authorized by law. This 143-year-old law embodies an American tradition that sees military interference in civilian affairs as a threat to both democracy and personal liberty.
True, the President and the Secretary of Defense and the four service secretaries (Army, Air Force, Navy, and Marines), are civilians; it was designed that way to avoid a military coup. The highest ranking generals are subordinate to their civilian leaders; should any of those civilians issue an unconstitutional order then that person would be subject to impeachment by the Congress. Checks and balances; our gov't was designed so that no person or group could take over complete control. Hope that helps.