I have imported from my Windows desktop program to Thunderbird Android App, but existing messages are not appearing, only new ones.

Is Thunderbird desktop connecting to your email provider using the Pop3, rather than Imap protocol? Pop3 downloads and stores emails locally on your desktop computer. Once downloaded, emails are typically deleted from the server. The Android app can only download emails on your providers server. Older emails stored on your computer would not be available. https://www.geeksforgeeks.org/computer-networks/differences-between-pop3-and-imap/