Technically anyone can install the developer beta builds if they have access to the developer beta profile, but a better choice for curious users who wish to try out iOS 13 and iPadOS 13 is by enrolling in the public beta testing programs. New beta updates to watchOS 6 and tvOS 13 are also available for users participating in those beta programs.
Separately, MacOS Catalina Beta 4 is available for Mac beta testers as well. IOS 13 beta 4 is available to iPhone and iPod touch, whereas iPadOS 13 beta 4 is available for iPad. Users enrolled in the iOS 13 and iPadOS 13 beta testing programs can download the latest versions now from the Settings app > General > Software Update. The corresponding iOS 13 and iPadOS 13 public beta 3 builds are also available to download now. Typically a new developer beta build is released first and soon followed by the accompanying build as a public beta that is versioned a number behind, for example iOS 13 beta 4 is usually iOS 13 public beta 3 but sharing the same build, in this case 17A5534f. IOS 13 and iPadOS 13 include many new interesting features for iPhone, iPad, and iPod touch, including a new dark interface theme, a redesigned Photos app, a new Find My app that helps you find your devices and your friends or family, new features to Reminders and Notes, new multitasking features for iPad, and much more.