Wash Sale Rules - ETFs and underlying securities

Hi All - like many people, i am taking some losses in technology investments.  What I don't understand is, if i get back in at lower prices, if it constitutes a wash sale.  For example, to take popular tech funds like ARKK and ARKW, if i sell one to generate a loss, but then within 30 days I buy the other, will that constitute a wash sale?  Also, if i had held individual equities that happen to be part of one of the funds (Roku, for example), and I sell Roku at a loss and within 30 days buy back in to ARKK, will that also constitute a wash sale?  I don't really understand what "substantially identical" means.   I need to be certain on this as it has major tax implications going forward for me, so please only respond if you are certain.  I appreciate your help!