Populate String field value in another field of list datatype

[Mehar Naaz1]

I have to populate value of a string field  'X'  into a field 'Y'  with List datatype in "Location" table.

'X' field have multiple values (comma separated) 

If 'X'  field value changes, 'Y'  field should have all the values of 'X' .

Then check whether that value is present in "Z" field in "Building table". 

if the value is present in 'Z' field in building table then store the value in 'Y'  field which is in "Location" table.

[Ankur Bawiskar]

@Mehar Naaz1 

very difficult to visualize without any sample script or screenshots.