Sync Panic Transmit keychain I would like to sync Panic Transmit keychain via iCloud. I know that Panic offers Panic Sync but I prefer to use iCloud keychain to sync FTP password.

In this page https://library.panic.com/transmit/new-mac/ it says that passwords are in /Users/USERNAME/Library/Keychains/ but I see different keychains.
Panic does not use its own keychain, but rather just the standard Login keychain, which will be located in the folder you were looking at. You're not looking for anything Panic/Transmit in that folder but rather just the Login keychain. You can open the keychain with Keychain Access to view the passwords within.