After successfully mashing your project, tap on the Save Movie (iOS) or the Save to Gallery (Android) button located at the bottom of the Share Mash page:
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This will save your mashed video in the Photos app (iOS) and under the Collections > On this device > 1SE in the Google Photos app for Android Google Pixel devices, or under Movies or Videos > 1SE for other Android phones.
To share it, take a look at this article.