How to insert a character between any several characters without knowing indexes?

This is what you want ?

StringJoin[Riffle[Partition[Characters[#], 2], "&&"]] & /@ {"x1x2", 
  "x1", "x1x2x3", "x3", "x4", "x1x2x3x4"}

(* Out: {"x1&&x2", "x1", "x1&&x2&&x3", "x3", "x4", "x1&&x2&&x3&&x4"} *)

lst = {"x1x2", "x1", "x1x2x3", "x3", "x4", "x1x2x3x4"};

You can use a combination of StringRiffle and StringPartition

StringRiffle[StringPartition[#, 2], "&&"] & /@ lst

{"x1&&x2", "x1", "x1&&x2&&x3", "x3", "x4", "x1&&x2&&x3&&x4"}

Alternatively, you can use StringReplace:

StringReplace[lst, d : DigitCharacter .. ~~ a : LetterCharacter :> d <> "&&" <> a]

{"x1&&x2", "x1", "x1&&x2&&x3", "x3", "x4", "x1&&x2&&x3&&x4"}

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