How did datatypes work?

[Piru]

Hello, Is there a good explanation of how data types worked?

http://en.wikipedia.org/wiki/Amiga_support_and_maintenance_software#Datatypes

Also was this the default way how files where handled/opened on workbench?

No

[Piru]

Yes, but is there any documentation of how this worked internally

You should be more specific in your questions then. There most certainly is.

and how I might develop a new datatype?

Googling does help.

http://aminet.net/package/dev/c/C_V44-DT