Multiple Databases 1 app?

I have an app that is designed for employees to easily access the companies policies. Other companies are interested in having their policies on the app as well. What I am wondering is there a way that when a user opens the app they can choose which companies policies they are viewing? To build a whole new app for every company is silly when there is already the framework, I just can’t figure out how a user can select which database. Its not as simple as having icons at the bottom for the individual company. Within each company they have their own policies, procedures, and forms.
Thank you for the assistance,

You could do that with security filters. Company A will have access only their own data. Please check these articles how Security filter works…

Thank you so very much for this information.