Getting Started. M4 Mac Mini with home folder moved to external.

[SudoArt]

Output of brew config

HOMEBREW_VERSION: 4.4.26
ORIGIN: https://github.com/Homebrew/brew
HEAD: f1041842141f20bc701b522d4371d7843d905580
Last commit: 4 days ago
Branch: stable
Core tap JSON: 27 Mar 18:38 UTC
Core cask tap JSON: 27 Mar 18:38 UTC
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_MAKE_JOBS: 10
Homebrew Ruby: 3.3.7 => /opt/homebrew/Library/Homebrew/vendor/portable-ruby/3.3.7/bin/ruby
CPU: deca-core 64-bit arm_donan
Clang: 16.0.0 build 1600
Git: 2.39.5 => /Library/Developer/CommandLineTools/usr/bin/git
Curl: 8.7.1 => /usr/bin/curl
macOS: 15.3.2-arm64
CLT: 16.2.0.0.1.1733547573
Xcode: N/A
Rosetta 2: false

Output of brew doctor

Your system is ready to brew.

Description of issue

Running my Home folder off an external is working great for me except when I try and use Ruby and Bridgetown on the external. My problem is that Homebrew tells rbenv where to put the ruby that I want to use. When I tell rbenv to run that ruby as global it doesn't work and I've realised that it's because Homebrew is putting rbenv and the rubies on the external.
I'm fine for Homebrew to use the Macintosh HD rather than the external for everything. I just can't figure out what I need to change or rather how I need to change it. From what I understand it's Homebrew's environment file or files that I need to change. I'm just not good enough to understand what exactly I need to change. I uninstalled rbenv and Homebrew yesterday and reinstalled Homebrew again after getting a whole crap load of errors when I tried to manually install Homebrew onto the Macintosh HD. I panicked and quickly reinstalled Homebrew to it's default location. Everything is fine now and I would just like to know what I need to change to have rbenv put the rubies onto the Macintosh HD rather than the external. I'm fine to use the installed HD as my ruby workplace.