making Catalina usb device

You also need the instructions in this Apple article for creating a bootable macOS USB installer (also contains links to various macOS installers):

How to create a bootable installer for macOS - Apple Support

You can only download a macOS 10.15 Catalina installer using a Qualifying Mac. A Qualifying Mac is one which is capable of running macOS 10.15 which is most Macs from 2012 - 2019. You can confirm a Qualifying Mac using the information in this article which shows which hardware is compatible with various versions of macOS.

https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

a Bootable Installer. 

The Bootable Installer can Only be performed on an Apple Computer 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of Catalina would have to be done on a computer that Qualifies to run Catalina.

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.

Alternative is, to gain access to a Qualifying Apple Computer from a family member, friend or associate.

How to get old versions of macOS - Apple Support