How is PC memory organized? It is accessed byte by byte or are there larger access units?

I have an application with an array of user type variables. I noticed that when a user Type variable is written to a file, all the subordinate variables are packed with no filler. How do these subordinate variables appear in memory?

On a mainframe, the order of the variables in an input/output record affects the amount of memory required. Does it have an effect for a VB program?

Suppose I have Type variable with a subordinate 9-character fixed string and some subordinate Doubles. If The string is first, is there unused space between it and the first double? Will defining the subordinate doubles first save some memory space?