If you are used to other operating systems you will no doubt be amazed that there is no official file management system in either iPhoneOS or iPadOS !
No matter what, files will have to be uploaded/downloaded using a cloud service.
N.B. These cloud services can be any of the popular ones like Google Drive, Dropbox, Adobe Creative Cloud, OneDrive, Box or iCloud {but this has restrictions} or any of the other similar services available. Whichever one you opt for you will need to install the corresponding App in order to use it.
This business insider article explains how to use Apps as a means of downloading files as neither of the devices use a traditional ‘downloads’ folder. You need to understand the purpose the file you are downloading is to be used for and then have an App to manage the downloading, storing and data retrieval.
This idownload blog article tells the same story and mentions using :-
(a) the Files App [already installed - see Apple Support page] to which you can add the 3rd party Apps
and
(b) the Documents 5 App [free to download from the App Store] that is invaluable for downloading files that are stored on a password-protected site.
This TechRepublic article explains how to use the Safari browser as another way of downloading files.
N.B. By default your Safari browser will use the Downloads folder but can be configured to point to an alternative destination.
This EaseUs article explains three different methods ** that can be used to delete unwanted files from your device.
** using the Files App, iOS Content Manager or via Settings
Click here to see a HowToGeek article that shows you everything you need to know.