You are not used to the way things are. This is both your super-power and your biggest pitfall.
That feeling of the new is a bittersweet one. Let's make it easier to enjoy.
You are about to join a new team. What do you do first? That is a question that many get wrong in my opinion.
The idea is that one is joining to contribute as much as possible and to disrupt as little as possible (at least at the start).
Here's my suggestion on how to approach your first months on a new team.
Start with observing. Notice how the team works, what are their practices, what are the team dynamics and learn from it.
When you start you do have the advantage of a fresh pair of eyes. That will allow you to notice things that the team cannot see any longer. Take note of that.
However, you lack context. This is the time to start gathering this context, asking all the questions, and start building your inner knowledge of the culture, practices and habits of your new team.
Ask all your questions – especially the stupid ones.
Your goal at this stage is to get up to speed on the team's work while building as much context as you possibly can.
Put in the work. Learn how to work like your team does and put in the leg work.
You were hired to help the team somehow. Understand your role and start contributing to it as early as you can.
At this point it doesn't matter if you can see a million ways that things could be done better. You need to help with the basics first. Especially if you joined a struggling team.
This will help you earn credibility with your teammates. No matter what your title is, it is easier to affect change when you earn your credibility.
Put in the work.
Make sure that you understand in depth why the team operates like it does.
I cannot stress this enough: Do not assume you know why. Ask.
If you have hopes of suggesting/implementing improvements and wish those to go smoothly you must understand the context. The larger the team the more important this becomes.
You can have a genius idea on how the team should do their API deployments. If you lack a key piece of why they do as they do, your genius idea will be quickly brushed off.
Understand the context. Walk miles in your teammates shoes (metaphorically :D) while taking note of what needs improving.
You will thank yourself later.
And now, after you've put in the work as your teammates do, after you understood in depth their practices, you can come back to your notes and ask yourself:
"What could we do better here?"
There's always something. If you've been diligent you have noticed a few. If the team is struggling you might have noticed many and need to pick your battles.
But remember, change is not always the same as improvements.
Always make sure to not conflate your opinion of better with it being objectively better. Understanding the team goal helps here.
A good way to think about it is: if something is indeed better you should be able to show data (measurements) on the improvements.
If there's no way to measure, then it is at best something that matches your preferences better. If that's the case, consider skipping it.
An important aside: people have different cultures and expectations. Don't expect that you will make friends for life in your new job, but do your best to be the one people want to be friends and work with. Maybe the friendship will come as a nice bonus.
All right, if you have more tips, catch me on X.