The file that you import into iMovie will be in whatever format in which the imported photo was originally recorded, if the format is supported by iMovie.
Here are the iMovie supported formats:
Current iMovie supports video formats Mp4/AAC, .mov, DV, HDV, Mpeg-4, M4v, H.264, HEVC H.265 (High Sierra and later). There might be others. A clip in one of the above formats should import into iMovie. There are no format settings for import -- it just automatically imports at the setting of the clip. However, not all Mp4 formats work with iMovie. It needs to be Mp4/AAC. AAC stands for Advanced Audio Coding. Also, the .mov format in general is a bucket that can contain varying types of formats and codecs, some of which may not be supported by iMovie. Fortunately, even if the video is in an unsupported format pretty near everything can be converted to a format that iMovie can handle.
iMovie supported audio formats include mp3, WAV, AIFF, AAC, m4A.
iMovie exports everything in Mp4 except when you select to export in Best Quality (pro res), in which case the export will be in .mov format with a high bitrate and a file size that is about 4x larger than an Mp4 export. iMovie will edit in the new HEVC H.265 format, but will convert to Mp4 on export.
-- Rich
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