How to dual boot Ubuntu and macOS without rEFInd?
If you employ all the security offered by the T2 chip, then you can not boot Ubuntu. Also, you can not boot the Ubuntu Live USB flash drive to install Ubuntu. So with secure boot enabled, you can not install Ubuntu.
If you lower the security, then you can install and boot with or without using rEFInd. Installing Ubuntu to the internal drive of the newest Macs may not yet be possible.
With rEFInd, the following happens:

The firmware boots rEFInd.
The user chooses Ubuntu from the rEFInd menu.
rEFInd instructs the firmware to boot Grub.
The firmware boots GRUB from the EFI partition.
Grub boots Ubuntu from a linux formatted partition.

Without rEFInd, the following happens:

The user holds down the option key at startup until the firmware boots to the Mac Startup Manager.
The user chooses Ubuntu from the Startup Manager. (By default, the label is EFI Boot.)
The shim boot loader is booted from the EFI partition by the firmware.
The shim boot loader boots Grub from the EFI partition. Ubuntu is also made the default operating system to boot.
Grub boots Ubuntu from a linux formatted partition.

There is also option with rEFInd to do a Grubless boot of Ubuntu. In this case, the following happens.

The firmware boots rEFInd.
A driver is installed by rEFInd that allows the firmware (and thus rEFInd) to read linux formatted partitions.
The user chooses Ubuntu from the rEFInd menu.
rEFInd instructs the firmware to boot Ubuntu from a linux formatted partition.
The firmware boots Ubuntu.

There are other scenarios for booting Ubuntu. The three given are fairly common.