Reformat MacBook Air mid 2013

i have a macbook air (13 inch mid 2013) and i want to reformat the hard drive and reinstall big sur on a clean drive. i followed the instructions on the site and it went through and reinstalled big sur but i only have 23 gig available on the drive and the capacity is 256g. So this is telling me it reinstalled Big Sur but left all the previous programs on the drive. How can I get rid of the programs and get a clean drive so I can give this away. Thank you.