If fed cuts rate, it means it's cutting federal fund rate right? then why do loans like credit card and stuend loan? those aren't like Mortgage rate which the fed cuts its rate by buying MBS
The Fed funds rate is the rate at which commercial banks can borrow reserves on the overnight market (see this explanation at Investopedia). As such it affects all other interest rates banks charge since when they can borrow more cheaply they can also lend money cheaply to consumers.
This affects among others also student loans (when they are provided by commercial banks - some places have government provided and subsidized $0\%$ interest student loans). However, of course if you already had a loan in a past it will only be affected if you took it at variable interest rate or if you are taking out a new loan, the fixed interest rate loans won't be changed retroactively.
The Fed doesn't cut mortgage rates by buying MBS. MBS didn't even exist until 1980s. The fed funds rates affects the rate at which banks can borrow from the fed, and hence the rate banks are willing to lend to people. Only in the case of the adjustable rate mortgage (ARM) does the fed rate directly affect the mortgage rate. Almost all mortgages rates are fixed at origination. Refinancing changes the rate, but by replacing one mortgage with another--loaning someone new money at a lower rate to pay off the old loan.