Editing php code (and sql) woocommerce form search

Hello,

I want to customize my woocommerce theme (in order to select multiple categories with keys words), or perhaps just edit the basic wordpress form seach ?

What I want to do is check multiple product cat (check multiple checkbox in the form) and to do that I think that I have to use multiple get value in the sql query (or).

So where I can edit the code ?

Thanks in advance.