How can I make copy paste generator like f.e. lingojam, from which I will can copy and paste text with all formatting 𝓵𝓲𝓴𝓮 𝓽𝓱𝓲𝓼.
I ask about this problem because I want to do it with another font.
You cannot do it with another font because it's not really formatting, it's rather replacing regular characters with other Unicode characters that look like the same letter in a different design.
For example, the 𝓵 is the character MATHEMATICAL BOLD SCRIPT SMALL L with code point U+1D4F5.
It is basically abusing the characters that exist in Unicode for a different purpose.
If the character "designs" that you are looking for exist in Unicode already, you can build such a tool very simply, just write code that replaces all occurences of the character l (U+006C) by the character 𝓵 (U+1D4F5) for example (for all the letters of course). If no such characters exist already, then you are out of luck.