About Subspaces Creation: Subspaces are created from an adept skill (Waijing), specific items, mind power.
Post-creation: Realms are created with their own "sun", sky, sea, etc (probably depending on what the realm looks like).
About: Subspaces are not true worlds, and have their limitations:
Paimon: Wow, is that all it takes? Then... we could build a whole city inside, couldn't we!
Tubby: Hmm... I doubt it. A golden-eyed adeptus explained this to me at some point in the past.
Tubby: He said that even though Sub-Space Creation is a product of adeptal power,
even that has its limits.
Tubby: This
world is not a true one, after all. It provides merely a moment of brief respite from the mortal realm, not a means of escaping it entirely.
Taking into account the realms that we see in the game, they all look different, which means that the person chooses the appearance of the kingdom at the time of creation (probably using the power of the mind).
This is due to the fact that Madame Ping is not seen with materials that would be needed to create the items she already has in the realm's po-creation, such as a sun, sea, earth, house, etc. In addition to Madam Ping asking for items that we have not seen in the realm, and would have nothing to do with it.
Madame Ping: Well... For starters, I require some Shimmersoil from the banks of
Dihua Marsh. Back in the day, it could only be found where the
Glaze Lilies thrived most profusely.
Which means that it's quite obvious that creating a subspace is different from creating the items inside, as you have to "prepare the materials" (have them) to create the items using the power of your mind, being materials they use to create the same object normally (such as wood to create a chair), different from subspace creation.
Not only does subspace creation not use materials like item creation (like taking water from an ocean to create an ocean in the realm), it also uses adept abilities (Waijing) and mind power, as far as I've seen none of that seems to escalate to AP.
And about the creation of items, in addition to not having anything to indicate that Traveler has created something like a city or something (taking into account the limit mentioned by Tubby, and nothing in the lore mentions this), there is still the fact that who has no reason to escalate to AP, Traveler just needs to get the materials, the creation of the item comes from the imagination/mind.