securitisation and tranching

John i have been wrecking my head trying to understand these lines(technical article). Securitisation is achieved by transferring the lending to specifically created companies called ‘special purpose vehicles’ (SPVs). In the case of conventional mortgages, the SPV effectively purchases a bank’s mortgage book for cash, which is raised through the issue of bonds backed by the income stream flowing from the mortgage holder. I dont understand who is transferring the lending? Is it a company(or is it a bank?) that creates an SPV, then sells its lending to the SPV? And the SPV gives cash to the company as a return.In return SPV gets the company's mortgage book from which it will receive mortgage payments. If it was the company that created SPV (then isn't the SPV effectively a part of the company? So if that is the case, where does the SPV bring cash from? Isn't it the company's cash that is going back to the company. Or is SPV another, different entity?