Where to focus on your first 90 days?
The first 90 days are critical for any new joiner. It is so because one can leverage "being new" to the organization to learn a lot, form relationships, and get to the rhythm of their co-workers. After the first 90 days, one is perceived and not being a new employee anymore and hence the expectations from their role start trickling.
The most important job during your first 90 days is to form relationships across the company. A company at the end of the day is people, teams are people, and dependencies are also often on people. The more people with whom we form dependable relationships we'll be working with directly and going forward indirectly the better. Dependability means they can count on you for getting something done, or simply make their challenges yours and invest time to find a solution and go back to them with it in a reasonable time.
The second most important job for you during your first 90 days is to learn about the platform and tools you'll be working on and with. Since post 90 days the expectation from you would be to deliver working with those same platforms and tools like your peers, the 90 days lead time provides for that opportunity.
Finally, the third most important point is to work with your manager to develop your desired "Growth Roadmap." A growth roadmap is specific to you and lays out all the steps that you need to take and goals to achieve for you to grow into the next step of your career. Towards this, you need to spend time with yourself and ask and determine where you want to be 1 year from now in your career. To this end try learning how your company does appraisals or provides feedback to you about you.
And soon you'll find yourself breaching the 90 day threshold and your peers starting to look at you as one of them "oldies" in the company, its relative but the sense sets in.