while doing the bill of the 2nd peron the bill of 1st person get in this 2nd bill..what am i doing wrong?????

Question

s={'football':800,'bat':890,'ball':560,'shoes':480,'boots':630,'jersey':470}
det={}
cust={}
def cus():
    nm=input('name:')
    ph=int(input('ph_no:'))
    det[nm]=ph
def buy():
    a=input('name:')
    if a in det:
        print('MENU:')
        for i in s:
            print(i,s[i],sep='=>')
        b=int(input('how many items needed:'))
        sum=0
        for i in range(b):
            c=input('what do you want:')
            d=int(input('quantity:'))
            cust[c]=d
            e=s.get(c,0)
            f=e*d
            sum=f+sum
def bill():
    g=input('name:')
    if g in det:
        j=f'''
                              BILL:
                            ---------
            name:{g}
            phn no:{det.get(g)}
            items:          quantity:        u_price:        price:
        ----------------------------------------------------------------'''
        print(j)
        if g in det:
            j=0
            for i in cust:
                t=s[i]*cust[i]
                j=t+j
                print(i,cust[i],s[i],t,sep='\t       \t')
            print('total amount:',j,'/-')
            
shop='''
        1.new customer
        2.buy product
        3.billing
        4.exit
        '''
while True:
    op=int(input(f'{shop}\nChoose your option:'))
    if op==1:
        cus()
    if op==2:
        buy()
    if op==3:
        bill()
    if op==4:
        break

Answer 1

1. Your variable names do not help to identify what you store in them. Use longer, more talkative names (e.g. customers, customer, total, etc)
2. Even worse, you reuse the same variable name to store different things in them (e.g. j is used to store what to print on the screen then it's used to store the total amount.

I think the main problem is that you put all the purchases into cust, regardless of which customer purchased it:

cust[c]=d

Separate purchases per customer then you should be fine.