I want to build a stronger relationship with my manager, but I'm unsure where the boundary should be when it comes to opening up to them. For example, I have some confidence issues which affect my work performance a bit: Is that something that's okay to share with them or is it possible that they'll hold it against me? I haven't interacted too much with my manager as I'm fairly new to the team, but so far, the signal has been positive.
Follow up: How can I ask my own peers about my own manager? (Is it something they/anyone would be comfortable sharing especially if u know your colleagues only for few months/weeks)
Follow up: How can I ask my own peers about my own manager?
This is definitely more sensitive and requires a deeper relationship with your peers before going into it.
One "trick" though is to not ask your peers about your manager and simply talk about your manager naturally instead. From there, your peers might respond by talking about their opinions of the manager. So if you're talking to your coworkers about work, maybe you can bring up something like, "Our manager just put me on project X, but it's been hard figuring out everything about X to ship something there." This opens the door for your coworker to talk about a similar experience they had with your manager (if they have one).
Another angle is to bring this up in the team selection phase (i.e. before you have joined the team) where it makes more sense. Back when I was switching teams at Meta, I asked the current engineers on the team I ended up joining (Instagram Ads) what they thought about the manager. I asked the questions from this video: Questions You Can Ask To Evaluate An Engineering Manager