Where do you get all the updates on Miles promotions and discounts?

Adding onto the comprehensive responses that you have received, I personally feel the Milelion Roar telegram broadcast channel (https://t.me/milelionroars) is very useful.

Milelion posts all (I think?) promotions almost in real time, as well as does a Weekly Deal Summary to ensure you do not miss out on any promotion. Best of all, it is a channel so no spams! Only the best stuff shared there

I would use milelion quite regularily but sometimes I just do a random googling to see if there is any promos out there :) Somehow it has been quite successful

You can join the facebook groups for those credit card companies & keep yourself updated for all the upcoming promotions.

I refer to 2 websites : milelion and mainlymiles

Because they provide weekly updates of the latest promotion