Hello Yahmose,
To see emails send to a hidden email address, you would just go into the Mail application > Inbox folder and open the emails the same way as an email that was sent directly to your actual email address. If you need to change the email address you have hidden addresses set to forward to, you can do that in Settings > your name at the top > iCloud > Hide My Email > Forward To section at the bottom of the page.
Create and manage Hide My Email addresses in Settings on iPhone
Use Hide My Email in Mail on iPhone
If you're not currently an iCloud+ subscriber, you may have been at the time you set up Hide My Email. The Hide My Email option will only in Settings > your name at the top > iCloud, when you're using iCloud+ (a paid storage plan). If you're just using the standard 5 GB storage plan, then the Hide My Email service is unavailable.
If you're supposed to be on the standard 5 GB storage plan but you have an option for the Hide My Email service, then you may want to check on your storage to confirm that you are actually on the 5 GB storage plan. Settings > name at the top > iCloud > Manage Account Storage.
Cheers!