On the screenshot you can see model inside, this line connect (unioned) objects, from outside it looks perfect, inside I have these connected artifacts, It works fine to select each face and delete it, but I tired after 15 min of doing it 
There’s a few ways- one option is a Remesh modifier. You can also enable the 3D Print Toolbox addon which has some tools for this
Hi. you can also select one face and Select linked.
Select Linked doesn’t work here
Remesh will kill perfect geometry outside, ofc I can do with super small faces, but object is quite big, and It would be hard for my PC.
(Dyntopo doesn’t fix it.)
I don’t have 3D print ToolBox, but I will try to find it on the internet. Thank you!
How do you achieve this result?
Perhaps there’s a better way of doing it so that you don’t have to remove these faces manually.
Could you upload (part of) your .blend file by any chance?
^^ I did boolean, after I found walls inside, I wanted to clean it, and I selected everything from outside to make inverse and delete inside walls, It worked, but left these things… 
I cleaned it by boolean so right now I don’t have saved file with this problem :<
That would clean the mesh by itself. 
I cannot reproduce anything similar as that, but if you still have that intersection edge selected, you can try to split the edge with ‘V’, and try again the ‘Select Linked’.
I was talking about a version of the file before the problem. Sorry, that wasn’t clear.
Did you use surfaces or volumes for the Boolean operation?
There is always a way, but sometimes the only way is manual work. Additionally, it depends on the tools in your tool belt. It’s difficult to offer a sound solution without seeing the actual mesh, as these types of problems are typically addressed on a case-by-case basis.
The edge is currently selected
In this case, separate the edges (Hot Key V) and select one (Hot Key L) to delete them… 