Am I right: there must be a build step such as Vite, as long as you see this code wherever: import ... from 'vue'

See this pull request before you read on, please.

Hence, this beginner underwent the same difficulty in 2019 as mine now if I am right. I think the reason why that error happend on him or her is because he or she used the wrong code import Vue from 'vue' without a build step.

Of course the new fearure importmap (at the third code snippet) allow us to run the code import ... from 'vue' in plain JavaScript without a build step.

Therefore, am I right with this conclusion:
Without a build step such as Vite (and except for using importmap), the code import ... from 'vue' can’t run, instead, we have to use const { createApp } = Vue (in the case of Vue 3)?

Let me know if I am wrong.

The import XXX from 'YYY' is just plain JS import syntax, and it can run in a web browser. But then you need to have a server sending back the JS file YYY, which you won’t have unless you set it up yourself. Using an import map you can tell the web browser to request another file than YYY when YYY needs to be imported, so this way all code importing vue will work, and the web browser will fetch the JS file YYY is mapped to instead (for example Vue hosted on a CDN).

So, a build step is not required from this point of view. But with a build step you can do more fancy stuff in your code, such as using single file components.