Ahora se renderizan los flatlist en el celular. Necesitaba tener dimensiones explicitas. Se las puse como un atributo dentro del tag de flatlist, immediatamente despues de abrit el tag.

screens/main/Home_page.js

 import React, {useState, useEffect, useRef} from 'react'
-import { Button, Text, View, StyleSheet, SafeAreaView} from 'react-native'
+import { Button, Text, View, StyleSheet, Dimensions} from 'react-native'
 import {FlatList, ListViewBase } from 'react-native'
 import {TouchableOpacity} from 'react-native-gesture-handler'
 import {List, Divider} from 'react-native-paper'
          //});
   }
 
+  const dimensions = Dimensions.get('window');
+  const screenWidth = dimensions.width;
+
   
   return (
       <ImageBackground style={styles.stdcontainer} source={require('../../assets/yellow-white.jpg')}>
-        <FlatList
+        <FlatList style={{
+             flex: 1,
+             width: screenWidth,
+         }}
           data={threads}
           keyExtractor = {item => item._id}
           ItemSeparatorComponent={() => <Divider />}
           </TouchableOpacity>
         )}
         />
-        <FlatList
+        <FlatList style={{
+             flex: 1,
+             width: screenWidth,
+         }}
           data={appointments}
           keyExtractor = {item => item._id}
           ItemSeparatorComponent={() => <Divider />}