Office 365 you pay for 365 days of use then unless you buy another year the software becomes unusable. Office 2019 is a normal software purchase, and will last as long as you keep it installed on the system you first install it on, just like normal software
Office 365 is the user-based subscription service powered by the Microsoft cloud. It includes the most collaborative, secure, and up-to-date features.
Office 2019 is a one-time purchase and does not receive feature updates after purchase. Office 2019 includes a subset of features found in Office 365, but it isn’t part of Office 365. Office 2019 will receive quality and security patch updates as required.