ELI5 GPS - how is GPS a free service?

How is GPS free? How is it profitable for companies to offer services like navigation without subscription fees or ads? Why did the US government open GPS up to the public? Why does GPS work even when underground and in buildings? Explain triangulation like i’m 5!
... we explain like I'm five
Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: gentlemantroglodyte, trick_individual_42, samkusnetz, thedankbank1021, bareback_cowboy, secretantworshiper and bbqroastgentlemantroglodyte, trick_individual_42, samkusnetz, thedankbank1021, bareback_cowboy, secretantworshiper and bbqroast.
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