The & operator joins text: =A2&" "&B2 puts a first and last name together with a space between. Text you type goes in quotes; cells don't.
CONCAT does the same as a function: =CONCAT(A2," ",B2). It also takes ranges, =CONCAT(A2:C2), but adds nothing between the parts. (The older CONCATENATE still works but takes no ranges.)
TEXTJOIN(delimiter, ignore_empty, text1, ...) puts the same separator between every part, and with ignore_empty set to TRUE it skips blank cells, so you don't get ", , " in an address.
Numbers lose their formatting when joined: 1234.5 joins as 1234.5, not $1,234.50. Wrap them in TEXT to keep a format.