You can use Baidu input method to type, and the specific operation method is as follows:

1. Download and install Baidu input method on your mobile phone, click the input box and open the input method.

2. In the open input method interface, click the symbol option.

3. In the symbol interface that opens, click the Russian option.

4. Then find the θ symbol and click it.

5. After that, you can successfully play the θ symbol on the Apple mobile phone.