We create collection of existing parameters using function 'def _format_arguments_from_db(...)' in a file '../pgadmin4/web/pgadmin/browser/server_groups/servers/databases/schemas/functions/__init__.py' @ line 454, and on client side we have backbone collection mapped with same id on line 304 (function.js)
We have written common function 'def get_types(...)' in a class called 'DataTypeReader' to fetch datatypes in a file '../pgadmin4/web/pgadmin/browser/server_groups/servers/databases/schemas/utils.py' and we have inherited from DataTypeReader in Function module's 'FunctionView' class to use it check line no. 120