You will have to make custom textInput component, something like this:
const [txt,setTxt] = useState("");
<Pressable>
<TextInput placeholder="Type Something..." onChangeText={(value)=>setTxt(value)}/>
<View style={{flexDirection: 'row'}}>
{txt.split(" ").map((text)=><Text style={{fontWeight:text == "bold"?'bold':null}}>{text+" "}</Text>)}
</View>
</Pressable>
Replace text == "bold"
with any value that you want to make bold.
Edit: if you want to make more than one word bold, group them in an array and update your condition like this:
const anArrayOfBoldStrings = ["bold","hello","stackoverflow"];
<Pressable>
<TextInput placeholder="Type Something..." onChangeText={(value)=>setTxt(value)}/>
<View style={{flexDirection: 'row'}}>
{txt.split(" ").map((text)=><Text style={{fontWeight:text == anArrayOfBoldStrings.includes(text?'bold':null}}>{text+" "}</Text>)}
</View>
</Pressable>