@ElleBelle great question! Condoms can be used when sharing sex toys like a dildo to prevent STI transmission between women. In other words: use a new condom on the dildo for each person being penetrated.
@Kit the sheets you’re thinking of are called dental dams! These can be used to prevent oral-genital transmission of herpes when anyone – male, female or nonbinary- is performing oral sex on someone with a vulva/vagina.
Lesbian, bisexual or queer women can also use a condom to avoid STIs whenever they have vaginal sex with a person who has a penis.
If you want to know more, check out this comprehensive article on safer sex & STI risk for women who have sex with women!