You might want to take some classes: you are doing well but you are missing some core abilities and functions.
You seem to be drawing in paperspace in all of your files: wrong! Take a class to learn how to draw in modelsapce at a 1:1 scale. paperspace is for printing/plotting only.
Your MeinA3 file is in a bit of a mess: it's not A3 in size, plus your titleblock is buried inside two different blocks (A$C7CBA445E and ALL are terrible block names and pointless to create) which also renders the titleblock useless to use as a template. Fix it by exploding it twice to get rid of the blocks.
If you want MeinA3 to be a block, then all the text needs to be ATTRIBUTE object types and not plan TEXT. You can't edit TEXT without exploding a block or always going into the block editor to fix. That's the hard way to do things. Very hard way to do things.
Your MeinA3 file has all objects on layer 0: that's fine usually but you might want to try placing it on it's own layer instead. Layer 0 should only be used for blocks you plan on placing on different layers and you'll never do that with a title block.
In your other files you also draw everything on layer zero: anotrher BIG mistake. Your objects need to be on their own layer, your dimensions should be on their own layer, titleblock should be on it's own layer etc. This is CAD, not paper drafting on a computer. Please take classes to real the basics.
Your text style in STANDARD: that's cute but a terrible idea. Why don't you own youtr text style and call it something unique instead. Using the STANDARD name for anything is lazy and can be very troublesome.
In MeinA3 you defined the text style as TECHNIC: in the other two files, your STANDARD text style is set to something else. The top file controls all the settings, not the file you INSERT into it. AutoCAD lesson #1 you should learn now.
You seem to be drawing in paperspace in all of your files: wrong! Take a class to learn how to draw in modelsapce at a 1:1 scale. paperspace is for printing/plotting only.
It will take some time to fix your drawings which I'm not able to do for you for free. Please consider taking some basic AutoCAD classes soon, even free online classes can help. Explore the free titleblocks provided by Autodesk in AutoCAD to learn how these are put together.
Good luck.