Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
The web-based versions of Excel, Outlook, PowerPoint and Word are free to use for everyone, but they offer limited capabilities (for an overview, see https://support.microsoft.com/en-us/topic/3e863ce3-e82c-4211-8f97-5b33c36c55f8).
Using the full version of Office requires that you get a subscription or purchase a perpetual (one-time payment) license. A perpetual license such as Office Home & Student will be cheaper; the downside is that it is only licensed for one user and one device. For more, have look at https://www.microsoft.com/en-us/microsoft-365/buy/compare-all-microsoft-365-products#.
If you cancel an existing subscription, you can still continue to use the e-mail address (access it online) and you can use the associated OneDrive storage. Remember that the free storage is smaller, though.