StringRiffle

StringRiffle[{s1,s2,s3,…}] creates a string by concatenating all the si, with spaces inserted between them.

StringRiffle[{{s11,s12,…},{s21,s22,…},…}] creates a string by concatenating the sij, and inserting spaces at the lowest level and newlines at the higher level.

StringRiffle[list,sep] inserts the separator sep between all elements in list.

StringRiffle[list,{"left","sep","right"}] use left and right as delimiters after concatenation.

StringRiffle[list,sep1,sep2,…] inserts separator sepi between elements of list at level i.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.