It choses the 'best' matching stream from all available ones, so there is always a fallback. Current order of properties for comparing candidates is: Format (if specified) / Resolution / Framerate (if specified) / Bitrate.
People with old systems can choose H.264 for performance reasons. People with modern systems can choose VP9 for best quality. AV1 is only used if you force it (for testing).
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For example, if I set 1440p H264 and play one of these 4k videos, will I get 1080p H264 or 1440p VP9?
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You get 1080p H264. If your system is capable of playing 1440p VP9, then setting H264 as preferred format is simply a bad idea. Just leave it at automatic. Right now I am not really interested in making things even more complex. Format is checked before resolution because an old system that can do 1440p H264 might not be able to do 1440p VP9.