Office and Microsoft 365 are the same thing now. Office was renamed and generally only used in reference to the disk-based version you might have bought in the past.
Windows doesn't ship with a copy of Office and never has. If you install a fresh copy of Windows then it will general install a trial copy of the Office/Microsoft 365 apps from the MS Store. This is just a trial though. It generally makes it clear if it is the trial version.
If you purchased your computer from an OEM like Dell or HP then you would have been given the option of buying a copy of Office as well. If you did that then the OEM would have provided you either a product key or a link to allow you to download the software. You need that information if they gave you a product key.
For newer versions of Windows however licensing is generally tied to your MS account directly. If you had Office installed before then you would have logged into it using your MS account. At that point the license would have been tied to your MS account. Go to your MS account profile and look under Subscriptions. If you see an MS 365 subscription then you are licensed for Office and can download from the account page (top left corner). When you start Office it'll want to log you in. Assuming you have a subscription then it should recognize that and you're done.
If you instead have a product key then download the version of Office you are licensed for and install it. Then start Office and log in. You should be able to enter the product key to associate the key with your installation.