Help request to better understand margin.

I understand how puts/calls work. but idk how margin works.

if i was to open a short put position on SPY for a $400 spy put, and lets say spy drops to that level and i get assigned to buy 100 shares of spy at 400. making me short 100 shares of spy at 400 each (ignoring premium for simplicity)

so now that im short 100 shares of spy, what happens with me and my broker? i know that if i had certain amount in my account in equity of other shares, i wouldnt get margin called. but how can i figure this out?

basically how can i figure out the relationship between my short positions and my overall equity?

lastly, if i had 10k cash in my account, and i get assigned 100 shares of spy to buy at 400 each, how will that work? is it a “buy all or be short the full 100” kind of deal? or will i buy short only 75 shares of spy, with the 10k cancelling out 25 shares of spy at 400 each?

in case this matters, i use TOS.