passwords not recognised after transfer

I’m getting a Joomla/Virtuemart shop transferred to WordPress/Woocommerce.
All the users details- name email password address have transferred, but WordPress refuses to recognise the passwords.
I was told in advance this would happen, but I don’t understand why.
I am definitely logging in with the correct username and password, I’ve even changed some of the passwords in the MySql database so that I definitely know what it is.

Does WordPress store the password in a second place in the database? or why are the passwords rejected?

It seems so frustrating that 1500 loyal customers are all going to have login difficulties and have to reset their password, when their passwords are definitely there in the Users table under password.