Why did an unasked for app download onto my phone?

Why did an app suddenly pop up on my phone that I didn’t download nor want? I didn’t download it for one thing I was downloading the chipotle app and this is social media one, so why did that happen? Is chipotle connected to social media or something? Besides that how can I stop that from ever happening again? I already have little space on my phone so I don’t want unauthorised apps just downloading on my phone and now potentially taking up space in my limited cloud storage.