For KDE Unstable Frameworks openSUSE Leap 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/KDE:Unstable:Applications/KDE_Unstable_Frameworks_openSUSE_Leap_15.6/KDE:Unstable:Applications.repo
zypper refresh
zypper install dolphin
For KDE Unstable Frameworks openSUSE Leap 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/KDE:Unstable:Applications/KDE_Unstable_Frameworks_openSUSE_Leap_15.5/KDE:Unstable:Applications.repo
zypper refresh
zypper install dolphin