What should I do when I bring the costume made by the marvelous designer to cc4 and put it on? I set it the way I saw it on learned it, and if I run cloth simulation, the costume falls to the floor as shown in the picture attached below
And how do I fix the phenomenon of the costume sticking out between my armpits perfectly like the one originally included in cc4?

Lets begin by saying that Reallusion apps do NOT have simulation engine for fitting and permanently baking cloth shapes.
What they have is an engine, which can only be used for interaction/animation at play time and mostly suits skirts, dresses, coats.. etc.

Any cloth imported into CC must already fit character perfectly (being simulated in MD, Blender.. etc).
Regarding armpits areas, it's always a problem and must be handled by either simulating in T-pose in MD,
or if simulated in A-pose it has to be handled by tweaking skin weights.
Aside from that you have to export cloth as "single object" , "thin" and "weld" from MD.

Here is a long workflow showing a proper simulation/export in MD with T-pose and A-pose (with fixing it later on).
For sim transformation from A-pose to T-pose in MD you got to have an iClone to create animated FBX.
(Sorry for speed and no comments).

Alternatively, you can import the simulated data in Marvelous designer into iclone8 or CC44 in alembic format.

Hello, thank you for the good video. I solved the primary problem by letting you know
Do you happen to have a video of applying the weight map to the clothes you made like the video you sent me????
If you have one, please do it!!!