ID of matrix printed instead of value
Technically the data you're seeking are under the first matrix block (not the first data of your .companyName - which is not valid)

{% set relatedCaseStudy = reviewFeatured.relatedCaseStudy.first %}
{% set firstCompany = relatedCaseStudy.companyInformation.first %}
{{ firstCompany.companyName }}



Note: If you plan to have only one company, think about a SuperTable field in 'static' mode (only one occurence).