Old mac, new blood

Ventura + Ubuntu on Macbook 2014

OpenCore Legacy Patcher

Follow instructions to install the latest compatible macOS on your old Mac.

The first step of ensuring whether your model is supported is by checking the Supported Models page.
Download and build macOS Installer

Make sure you change the target model in settings if building from a different mac.
Run the OpenCore-Patcher.app
Reboot and boot OpenCore

Use the Disk Utility tool first to leave a blank partition for Ubuntu installation later.
Post-Installation

Once you’ve booted into macOS for the first time, you will see the below message after a few seconds. It lets you install OpenCore to your internal drive, allowing you to boot without USB.

Have a read on OpenCore Multiboot. One disk - multiple OSes senario is picked for booting Linux on the same machine.

Download Ubuntu Desktop
Use balenaEtcher to flash the image to a USB drive
Boot into Ubuntu from the USB drive
Follow usual steps until custom disk partition

Format the blank partition to EXT4 (or whatever other FS you prefer) and make sure you choose the same EFI partition for where the bootloader will be installed (this should NOT “delete” OpenCore in any shape or form) and install.
Install as usual

The easiest method is using OpenLinuxBoot described in Dualbooting with Linux.

Mount EFI Partition with Terminal. See explainations here

# find the EFI partition
diskutil list 

# mount the EFI partition
sudo diskutil mount /dev/disk0s1 

Copy the ext4_x64.efi driver to the EFI partition under EFI/OC/Drivers/. You should also see the OpenLinuxBoot.efi driver already included.
Edit EFI/OC/config.plist Online with galada.gitee.io/opencoreconfiguratoronline
- UEFI -> Quirks -> RequestBootVarRouting = YES
- Misc -> Boot -> LauncherOption = Full
- UEFI -> Drivers -> Add ext4_x64.efi

Replace EFI/OC/config.plist with the edited one, unmount the EFI partition

 # unmount the EFI partition
 sudo diskutil unmount /dev/disk0s1 