Hi Mel,
Boostrap 5 is default framework for Helix ultimate and Joomla 4 as well.
If you want you can customize template code, but it's on your hands only.
Edit:
templates\shaper_estate\html\layouts\libraries\cms\html\bootstrap.php
plugins\system\helixultimate\overrides\layouts\libraries\cms\html\bootstrap.php
and remove link code to "bootstrap.min.css" file if this may help you somehow.
OR
Remove all content (CSS) from that file: templates\shaper_estate\css\bootstrap.min.css - but keep the file. Then bootstrap 5 styles from template would not be used, right.