What keys are bound to what shortcuts by default is in some ways an unrelated question, since as I said they can all be configured to the user's preferences. Also an open question: whether a selection-change keybinding also moves the Playhead, and to where. Whether the selection-change is a separate keybinding, or is implemented by having Prev/Next Marker jump to the adjacent clip after reaching the bounds of the current one, is up for discussion. not as useful as they could be, in practice. Jump To Start/End move outside the selected clip without affecting the selection, so after using them Prev/Next Marker are still constrained within the same selection. Prev/Next Marker will only move within the bounds of the selected clip. I do still think there should be a shortcut method to move the current clip selection to an adjacent clip. OpenShot's keybindings are reconfigurable for that reason. But you can rebind those keys to Prev/Next Marker in the Preferences, if you'd like to use them for that purpose.
Certainly, for quick one-handed video scrubbing it's convenient that you can trigger playback right from the arrow keys cluster. There are probably reasons for that, or at least some users who currently rely on one or both of them. ↑ and ↓ are currently bound as alternate "Play/Pause Toggle" keys, true. I recommend ARROW UP and ARROW DOWN (which strangely currently start/stop playback like space) to move left or right to the next start/end of a clip or transition or anything on any track. Those will take you to the extremes of the project. In addition to Prev/Next Marker, there's also Jump To Start/End, which are bound to Ctrl+ Home and Ctrl+ End by default. In #1363 I suggested that keyframes be added to the list of "things treated as a marker", so that Ctrl+Left/Right would let you quickly skip among all of. (Maybe you reconfigured them to Shift+ ← / Shift+ →?) They will also jump to the start/end of the currently-selected clip, which are treated as markers. Streaming media companies like Netflix and Hulu make deals with Roku to have their apps featured on Rokus remote controls, but theres an inherent flaw with this model: the buttons become useless if the app no longer exists.
What you're probably seeing are the effects of Ctrl+ ← and Ctrl+ →, which are "Previous Marker" and "Next Marker" respectively.
REBIND ADOBE PREMIERE KEYBOARD SHORTCUTS INSTALL
Shift+ ← and Shift+ → have no effect in my OpenShot 2.4.1 install with default keys. Not just undocumented, they're not configured as default keybindings. The only way to move quickly on the timeline via shortcut as far as I can tell is SHIFT+LEFT ARROW KEY and SHIFT+RIGHT ARROW KEY for moving to beginning/end of the currently selected clip.