The main difference between putting together data from two data frames using concatenation and using merging is that:

Select one:

a$.$ merging can only be applied on data frames created from SQL databases.

b$.$ concatenation only works when all rows match between the two data frames.

c$.$ concatenation simply 'copies and pastes' data while merging matches records based on a key.

d$.$ there is no difference, they can be used interchangeably.