[Vuejs]-What should I edit to fix this design?

Wrap your video tag inside the v-row; this will take the whole row for video and then the next row for your Select file button.

Here is the example code:

              <v-row>
                <video id="id" controls="controls">
                  <source id="src" />
                </video>
              </v-row>
              <v-row>
                <v-col class="d-flex justify-center" cols="12">
                  <v-btn
                    class="accent-3 blue mb-1 mt-6"
                    dark
                    @click="pickFile"
                  >
                    Select File
                  </v-btn>
                </v-col>
              </v-row>