In obsidian, a document can be viewed in either edit mode or reading view. In edit mode, you will see basic markdown rendering, but certain things, like code syntax highlighting and embedded files, will only show up in reading view. Most notably, any images you embed will only be visible in reading view.
To toggle between reading view and edit mode, click the reading view button in the title bar. In edit mode, it will appear as a page; in reading view, a pencil. If you `Cmd/Ctrl`- click, the reading view will be opened in a [[Linked pane]]. You can also toggle Reading view by hitting `Cmd/Ctrl-E`, but this hotkey can, of course, be changed.
If you want to default to opening in reading view rather than edit mode, a toggle for that can be found in the [[Command palette]]. You can also set a [[Use hotkeys|Keyboard Shortcut]] for toggling between default states.