How do I update to macOS X 10.3 on PowerPC G4.

I'm sorry, but that is not valid. Yesterday, I've recently installed macOS 10.5.6 Leopard on my PowerMac G4 using a USB Flash Drive. And the Mac is not Intel Based either. Not to mention, Before I did any of this, my PowerMac G4 MDD was running macOS 10.2.8 Jaguar, so I've successfully skipped 10.3 and 10.4 and went straight to 10.5 and nothing is wrong. My boot drive is working, AirPort WiFi is also working, USB and NVIDIA graphics card is working, and I can actually watch YouTube. Mac OS can be installed on PowerPC's via USB's.

You will need to acquire the Panther (10.3) upgrade instal discs - I still have mine which is a set of three. There is no option to download the Panther instal discs and never was. The updates you have seen are small downloads - available at the time - and were applied to an existing system upgraded by a disc.

Panther was updated nine times to bring it to 10.3.9 and you can still download the combo updater for that here.

Having said that, I understand your PowerMac is capable of running 10.5.8 (Leopard) but the same deal applies - you'd have to get the instal disc then update to 10.5.8 here.

Is there a way to burn an macOS 10.3 Panther bootable ISO or DMG file to a blank disc? If so, how do I do it? I'm the same person but different account. I heard that you can flash a macOS X Installer on a USB flash drive and install the sequence by holding the down the option(alt) key.

Is there a way to burn an macOS 10.3 Panther bootable ISO or DMG file to a blank disc? If so, how do I do it?

As Apple never put up an open-source version of Panther online, and any you find for download are always pirated and often loaded with malware, no. Also many people who paid money for a pirated download iso or dmg found that their computers would not boot from any burned optical disk containing the file.

Apple Support page lists downloads of macOS, but it only lists macOS X 10.3

Those are updaters, not the base system.

I heard that you can flash a macOS X Installer on a USB flash drive and install the sequence by holding the down the option(alt) key.

A PowerMac MDD is not bootable from USB. Firewire, yes. Intel Macs are bootable form usb devices.

If you cannot find a full retail install disk on Amazon or another trusted source (auction sites are not trusted sources for Mac system software), then I recommend contacting a Mac Users Group. Many have legit copies of older system software archived

https://appleusergroupresources.com/find-a-group/

Well, you found one of the few situation where USB booting works. It does not on my Late G4 MDD. USB boot has been hit and miss and officially limited to a very small number of pre-Intel Macs. I am glad it worked for you.

Thank you, I'm glad too.!