Hiring for Dec grads started?

For which companies has hiring for Dec grads started? Amazon and Google reached out already so probably fb apple Microsoft and the likes have started as well. But what about other companies like Finance and fintech? Unicorns? Non big 4/5 like oracle, IBM? I'm struggling to get callbacks from these. Zero callbacks after applying online and even a couple straight rejects, so I'm not sure if its early or I need to find referrals. What is strange is Amazon and Google contacted me without even applying. Do I need to wait till Aug/sept to apply or change the way I'm applying?