You will be visiting your best friend and their whofamily who will be hosting you; ensure they can be contacted and will verify that statement.
Your parents paid for your RETURN ticket as a gift; ensure they can be contacted and will verify that statement.
Best to have their #'s on paper so you can keep your device locked and off.
Have proof of sufficient amount of funds on you / access to in the event of of an emergency (£500 is reasonable).
You recently graduated but are presently pandemic-induced unemployed.
At Immigration, clearly state you wish to visit your friend in Nebraska for 3 weeks. Answer the questions asked honestly and as succinctly as possible. Do not elaborate unless you are asked for more information. Be polite, keep a neutral expression, relax and breathe normally.
None of the above should make them question you further or make you inadmissible.
As others mentioned, don't pack excessively or inappropriately, dress appropriately, don't carry a resume in person or electronically (there's a remote chance your device might searched; evidence of looking for work in resident country: good; internationally: not good).