Is TinyDB capacity somehow limited or it is limited the only with device storage space?
Is it possible to build an apk with partially pre-filled TinyDB so that when a user apk downloads and installs, the application can with a database query use that data?
It is always good to search before asking any question.
You could use a csv file as an asset to fill your TinyDB when your app starts the first time.
I'm a complete layman. How csv-fail will created? How it to put along with an apk? How to use a csv file as an asset to fill your TinyDB when your app starts the first time?
PS. And besides, I'm worried that for the test I tried to put 300 characters long text in the CloudDB tag, it wrote it down and the value below it, but I can't delete it. The value was deleted, but ClearTag does not work on this tag.
yes
what about reading a bit in the community... especially the links @Peter already provided...
try the extension from @Juan_Antonio to import a prefilled TInyDB from the assets on first run of your app
A very good way to learn App Inventor is to read the free Inventor's Manual here in the AI2 free online eBook http://www.appinventor.org/book2 ... the links are at the bottom of the Web page. The book 'teaches' users how to program with AI2 blocks.
There is a free programming course here http://www.appinventor.org/content/CourseInABox/Intro and the aia files for the projects in the book are here: http://www.appinventor.org/bookFiles
How to do a lot of basic things with App Inventor are described here: http://www.appinventor.org/content/howDoYou/eventHandling .
Also do the tutorials Our Tutorials! to learn the basics of App Inventor, then try something and follow the Top 5 Tips: How to learn App Inventor
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.
I need to explain a little so that you can understand why it is difficult to find such a specific situation in these voluminous materials.
Namely, I program an app that, depending on its use, stores unknown data in a structure I don't know, but it needs to be put into google play later with the app so, that the user who downloads the app and starts using can continue from the created level.
So I can't put that data in a file myself. Is it possible from installed application created database to back up a csv file and restore it to the original database later? How to upload csv file in play console with apk or aab?
Get all TinyDB Extension by Juan Antonio to backup TinyDB as Text or JSON format
TinyDB Backup Extension by Atom_Developer
taken from the extensions directory App Inventor Extensions | Pura Vida Apps
EDIT: see also
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by Taifun.