The most common way is a user-defined function. This works by taking advantage of the "formatting codes" that Google Sheets offers for numbers (more on formatting in this help article).
To make your own formatting code, follow these steps:
Go to Cell > Alter Cell Format > Define New Format... Fill out the name and then put the format code below in "Expression" (see screenshot):
This concatenate function could be expanded by adding an additional number input field. Make sure to include this changes when constructing cells for each column. The evaluated expression would still remain as shown above. Alternatively, the user could merge cells to create a wider input range.

You might be also interested in reading about how formatting columns of numbers from Excel or Google Sheets automatically turns the number into something more readable like Euros or Dollars (check out Ankit's post here)