How do I get rid of Apple's system fonts from external booting drive?

But these fonts are impossible to delete.

You must be trying to remove the fonts on the same drive or partition you are currently booted to. You can't do that. Also to clarify a bit, drives are not memory (RAM). They are storage devices.

What you need to do (without resorting to using Terminal), is install Catalina on any other drive or partition you want. It can be on the same drive or an external. Doesn't matter which. Here's an example:

Catalina (I've given that partition the same name as the OS) is what I'm currently booted to. "Update" is how any Big Sur drive appears in Catalina or older. But let's pretend it's another installation of Catalina.

So, if I want to modify the main Catalina startup drive, I would restart the Mac and boot to Update. Once there, I can modify Catalina any way I want. You simply move through the folders the same way you would on any other drive or partition.

First things first. Always make a copy of every OS installed font to another drive or folder before removing anything. If you later find you need any of the fonts you're about to remove, you can always activate the copies with your font manager.

Then delete the unwanted fonts from the non-boot/startup drive. All you need to do that is enter your admin password.

Also, be very careful what you remove. If you overdo it, apps will not display correctly if you remove a font they rely on for display. Or, you could make the Mac unable to startup. In Catalina (or any version of macOS), you absolutely cannot remove certain fonts. Some are for the system itself, others are common fonts for the web that should always be available.

For Catalina, DO NOT delete any of the following:

/System/Library/Fonts

Apple Color Emoji.ttc

AppleSDGothicNeo.ttc

Avenir.ttc

Courier.dfont

Geneva.dfont

Helvetica.ttc

HelveticaNeue.ttc

HelveticaNeueDeskInterface.ttc

Keyboard.ttf

LastResort.otf

LucidaGrande.ttc

Menlo.ttc

Monaco.dfont

PingFang.ttc

SFCompactDisplay.ttf

SFCompactRounded.ttf

SFCompactText.ttf

SFCompactTextItalic.ttf

SFNS… (all other San Francisco fonts)

Symbol.ttf

Times.ttc

ZapfDingbats.ttf

/System/Library/Fonts/Supplemental

AppleGothic.ttf

Arial.ttf

Arial Bold.ttf

Arial Italic.ttf

Arial Bold Italic.ttf

Arial Black.ttf

Arial Narrow.ttf

Arial Narrow Bold.ttf

Arial Narrow Italic.ttf

Arial Narrow Bold Italic.ttf

Comic Sans MS.ttf

Comic Sans MS Bold.ttf

Georgia.ttf

Georgia Italic.ttf

Georgia Bold.ttf

Georgia Bold Italic.ttf

Impact.ttf

Tahoma.ttf

Tahoma Bold.ttf

Times New Roman.ttf

Times New Roman Bold.ttf

Times New Roman Italic.ttf

Times New Roman Bold Italic.ttf

Trebuchet MS.ttf

Trebuchet MS Bold.ttf

Trebuchet MS Italic.ttf

Trebuchet MS Bold Italic.ttf

Verdana.ttf

Verdana Bold.ttf

Verdana Italic.ttf

Verdana Bold Italic.ttf

Wingdings.ttf

Wingdings 2.ttf

Wingdings 3.ttf

Webdings.ttf

You're welcome. 🙂

Something to be aware of. If you upgrade to Big Sur, all fonts installed by the OS are immutable. There is no easy, or even somewhat easy method to remove them. While a good number of the fonts are disabled by default, Apple has many unnecessary fonts (for an English speaking user) on, and you cannot disable them. Like all of the Noto Sans fonts, Myanmar and others.

While your are booted into your external memory OS, can't you see the drive containing your main OS and just double click on it and navigate to the right folder and then drag the files to the trash?