I recommend using Expo's Crypto: https://docs.expo.dev/versions/latest/sdk/crypto

My use case was something like this:

global.Buffer = require('buffer').Buffer;

import { entropyToMnemonic } from 'bip39';
import * as Crypto from 'expo-crypto';

export function generateMnemonicList(): string[] {
    const entropy = Buffer.from(Crypto.getRandomBytes(32));
    return entropyToMnemonic(entropy).split(' ');
}