itch.io

"Create APK file on an android device, without using PC"

There are online services for converting a zip archive html to apk, but I can’t recommend a specific service. there are difficulties with the publication of such games in GooglePlay

"how to make smooth transitions of character sprites?"
Yes, in the sprite settings there is an "animation" section, in which you can set the "time transform" and for the specified time there will be a smooth animation of changes between the same sprite objects. You can do this using "Opacity" option and two sprites.

"only one folder is needed, in which all the resources are located? Can't split into folders?"
There can be any hierarchy of files in the working folder, but the editor will display all files without folders, I understand that this creates difficulties, but this problem will be fixed in the future.