1. Servers. I've always 'pushed to heroku' without really knowing how servers worked.
2. Git. I've always just used gitlab or other git hosting services without knowing how to build it.
3. File hosting. I've always just uploaded files without knowing how files are processed / stored.
4. Email. SMTP is like a big unknown, I've never really cared about how emails worked.
5. SSL. This was always done for me, I've never had to create and manage my own certificates.