They usually go by what the dominant strain was in the southern hemisphere during their flu season, and visa versa. Given that flu has been so suppressed in the past year as a byproduct of all the COVID-19 controls then I have to wonder how well they will be able to predict it this year.As I understand it, the flu vaccine for the year is made on something of a 'best guess' basis. They never know for sure which is going to be the dominant viral strain for that season, so they have to take a bit of a stab at it at the start of the vaccine production lead time.
I mentioned on one thread or another that several companies are working on combined COVID-19 and flu shots and hope to be selling those some time in the near future, although not in time for this flu season.
Flu shots based on the same technology as COVID-19 vaccines, whether separate or combined, are expected to be a lot more effective than the traditional flu shot. It will be interesting to see how that works out when they are available.