Clarification on an example contrasting the “BE going to ɪɴꜰɪɴɪᴛɪᴠᴇ” future with the “BE ᴠᴇʀʙ‑ing” future

Solution 1:

For the present continuous to be used as future, a fairly specific future time needs to be stated (i.e. "someday" would not work). It doesn't have to be so specific, just not totally vague like "someday."

For the "going to" construction, it does not matter if you mention a future time or not, as future is implied. Also you can mention a future time whether specific or not ("someday" works fine).

I assume They are going to meet again soon. is "Going to" future because "soon" is not a specific date. Correct?

"They are meeting again soon" would actually be fine too, but is not likely to be used often. But "They are meeting again someday" would not. There it would need to be "They are going to meet again someday." Or, of course, you can use the explicit future tense: "They will meet again someday."

A phrase I think is commonly used that is present continuous as future is "I'm moving soon" or "I am moving soon," like in "I'm moving to Dallas soon." So you can use "soon" with present continuous as future.