The places  where downloaded eBooks are usually found:

On Desktop Computers (Windows, macOS, Linux):

Default Download Folder:

Windows: Downloaded files usually by default appear in the "Downloads" folder. Access this by opening File Explorer and locating the Downloads folder.
macOS: The default download location is the "Downloads" folder within your user directory.
Browser-Specific Settings:

Some browsers let you change the default download directory. Check your browser's options page to see if you have set a different default.
Mobile Devices (Android, iOS)

File Manager App

Open File Manager on your device (or the equivalent app).
Look for a folder called "Downloads."

Browser App

Check the browser's download history or settings to see the eBook you downloaded.
The below methods are specific to Ebooks from Online Libraries and Platforms:

Overdrive: Downloaded eBooks from Overdrive might be stored in the default download folder of your device or within the Overdrive app itself.
Amazon Kindle: Store your Kindle eBooks within the Kindle app or on your Kindle device.
Google Play Books: Google Play Books downloaded eBooks usually are found within the Google Play Books app.
If you are unable to locate your eBook, try these extra tips:

Utilize your device's search feature: Search for keywords that refer to the eBook title or author.
Check cloud storage services: If you have a cloud storage like Google Drive or Dropbox, the eBook may have synced into your account.
Check your browser's downloads history: Browsers keep track of the files downloaded within the last time.
Reach out to the provider: If you downloaded the eBook from a particular site or website, you may reach out to their helpdesk and ask them for assistance.
By the steps above, you will therefore find your downloaded eBooks.