SphinxSearch be understood this way?

hypothetical premise:
A Weibo platform, posts are stored in Mysql, search using sphinx search.

1) under what circumstances will the rt index be used? can it be understood that when you publish Weibo, the program writes to Mysql and inserts it into Sphinx"s rt index? Or is the program just responsible for writing to MYSQL, and then crontab periodically performs indexer-- all-- rotate updates to the Sphinx index? But this way the rt index has never been used?

2) what is the data in the rt index to clear, and does it need to be cleared? Or will the contents of the rt index be automatically cleared each time Sphinx is restarted?

3) how to update to MySQL when new record data is written to Sphinx. Can only execute indexer-- all-- rotate to update the data of MySQL to Sphinx? However, there will be a lot of delay, resulting in data delay. Or, on the contrary, write to the Sphinx rt index first, then read it with a script and then write to MySql?