Which property has to be used to fill the columns created_by, updated_by etc in the table when the user uses the customized form to enter the data in the table?

get application property (username) to get the connected user.

To populate Created_by and Updated_by columns in your table you can use USER to get the current user id.

In pre-insert trigger by using 'fnd_standard.who' packagewe can insert data into these two columns in Oracle forms.

By using FND_STANDARD.SET_WHO in PRE_INSERT and PRE_UPDATE trigger, 5 who columns can be populated. But this work in Oracle Application form customization.