Setting up maildrop

I keep reading here & online, to go into iCloud Mail. I don’t understand. I have an iCloud email account but it’s just one of many I access via Mail on my iMac or iPhone. The others are gmail accounts. If I go into iCloud in settings, I can’t find anything for MailDrop. 🤷🏻‍♀️

My problem is I’m sending a file I don’t want to compress & its 3.42GB. I’m sending from my iMac w a gmail account to the same gmail account (which I do all the time) so I can open it on my phone. But I’ve sent it 3 times (1x as regular email), 2x as MailDrop. None of the 3 have ever come into my inbox OR shown up in my sent folder.

What am I doing wrong?🤦🏻‍♀️

I'm on iMac vs 10.11.6 & my iPhone is 6S+ iOS 13.4.1 Thank you!!