A bootable USB installer is exactly what it sounds like. A USB drive....usually a USB stick that has been setup up with the macOS installer on it. You actually provided a link to the instructions to create a bootable macOS USB installers in one of your previous posts. However, a bootable macOS USB installer can only be created on another compatible Mac since your Mac is currently not booting normally. For you, you would need access to another Mac generally from 2007 to mid-2020 which would allow you to create a bootable macOS 10.11, 10.13, or 10.15 USB installer (depends on the exact Mac). If you don't have access to another Mac from 2007 to mid-2020, then a USB installer is irrelevant and can be ignored.
Internet Recovery Mode (Command + Option + R) is just fine as long as you have a good Internet connection....seems like you were already able to boot into Internet Recovery Mode IIRC. You can access Internet Recovery Mode by holding down Command + Option + R immediately after hearing the startup chime. You may be prompted to connect to your WiFi network if you are not using an Ethernet Cable for the network connection.
Both Internet Recovery Mode and a USB installer will do the same thing although a bootable macOS USB installer can be a bit faster & more reliable, but both can do the job.
For reference you can review this Apple article which explains how to boot into Internet Recovery Mode (there are three options, use the one I mentioned for best results). And follow my instructions for using Disk Utility for properly erasing the external drive you will be using.....Apple's instructions for erasing a drive in this article may not always work especially for an external drive, plus it depends on which version of macOS installer you end up booting when booting into Internet Recovery Mode. Some Macs may only boot to the online installer which originally shipped with the Mac from the factory...in that case you will need to review my instructions in a previous post on how to properly prepare the external destination drive using Disk Utility. Scroll down to the section "If you are using any other Mac" as that is the section which applies to your older Intel Mac:
How to reinstall macOS - Apple Support
You can tell which version of macOS you are booting by looking at the utilities screen once booted into recovery mode. One of the options is "Install macOS xxxxx", where the "xxxxx" tells you the version of the installer (aka the version of macOS which will be installed).
Sorry this is so complicated, but Apple has made lots of changes over the years and some Macs will behave differently regardless of the special keys used to boot the Mac. And the instructions in Apple's recent articles are focusing mainly on the more recent versions of macOS forgetting that many people may be using older computers & installers.
As long as you can boot to a macOS installer and you only erase the external drive and only select the external drive as a destination, you can experiment as you wish or need to. Removing the internal SSD would ensure that you don't accidentally erase the internal SSD by taking out all risk in experimenting with installing macOS to an external drive.
Good Luck.