Webapplication with SQL Server for more companies

Does anyone know how to setup and architect a database that is ment for one
application but for more companies. The data of each company must be
protected and should not be visible for other companies. What is the best way
to do this? I rather don't want to use queries and indexing the main database
because i think it is not very secure. Is there a possibilty to switch from
database to another with the same architecture/tables? Or is there a
possibility to create more than one instance, private for every company. I am
new to build applications with this demandings. Thanks for your attention.

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Bartman