nick_h2, thanks for taking the time to respond.
Yes, I think I understand the various methods of receiving verification codes for two-factor authentication. In this particular case, I set up an online account with just a username, which is an iCloud email alias, and a randomly generated password. Later I decided that I should add my iPhone number to the account so that I could get text notifications from this particular account. This account requires a verification code in order to make any changes to a personal profile. So I requested to add a mobile number and got the "request a verification code" screen which said that a code had been sent to my username, i.e., my iCloud alias email address. But nothing happened. I clicked the "resend code" button multiple times but got no code. In one instance, I got just the title of a verification code email, but there was no content and it disappeared from the inbox almost immediately. After this happened, I checked my junk and trash folders, but there was nothing there. I also sent an email to the alias address, and it worked fine.
The next day I decided to try again. I requested a verification code and got nothing. I clicked "resend code" and got an email to the alias email account that had a title but no content. I clicked the third time and finally got an email with a verification code. I used it and was able to make changes to my online account.
Bottom line, I'm all set, but these glitches make me leery of an iCloud alias; however, as I said before, emails to this address from private sources have all worked fine. So I guess I'm good.
Thanks again for your reply,
dl68