Seniority generally shouldn't matter that much when giving feedback. You should respect them, their way of working and their opinion either way, and try to balance that with trying to have a high-quality and consistent codebase. With someone more junior you might insist on doing something because you know better (or that's how the team does things), but you should also try to explain why you're suggesting that, to both convince and teach them, which makes it rather similar to how you should interact with everyone else.
Typically code is commented on during code reviews / on pull requests. A 1-on-1 meeting might more make sense if you're looking to discuss the code in more detail, especially to understand it, or if you're mentoring them.
But this could be vary across different workplaces.
When thinking to which degree to criticise the code of others (or doing anything else, for that matter), I would suggest thinking of a new job roughly in terms of the 3 phases listed below. There isn't necessarily a clear line between them, and you may get to changing things quite quickly if you're hired into a more senior position, but it is a good basic way to think about things (and the fitting in phase is sometimes neglected for those hired into senior or management roles).
1. Fit in
Figure out how the team works. Then do things the way they do them.
What coding standards do they follow?
How strictly do they adhere to these standards?
How do they do code reviews?
How are reviewers chosen?
Can anyone review anyone's code?
To which degree do they comment on coding standards during reviews?
What's the policy/stance on refactoring code?
How open are they to changing how they work?
If they're open to it, and you can make a convincing case, you could potentially jump straight into changing things.
These are all questions you can ask, in some form or another, at any point from your first day, or even during the interview process if you really want. You can also find out some of the answers by just checking what people do.
It should always be okay to at least point out a bug or ask how code works or why they made certain decisions, but some workplaces may be exceptions, and anything beyond that depends on the workplace.
If you're having trouble judging this when asking about it or based on what others do, you could leave a review comment here or there and see how that's received. Start with the most significant issues. If it's received poorly, tone it down. If it's received well, you could comment on a few more things.
There's also the chance of finding the way they do things to be much better than how you are used to doing things, or they may have some constraints that make something that would otherwise be good impractical.
2. Prove yourself
You're not going to convince anyone to follow your subjective ways unless you've proven you know what you're talking about.
So prove yourself by doing your job well, writing good code, architecting good solutions, giving meaningful review feedback, mentoring others, getting promoted, etc.
3. Try to change things (if you want)
If you're not happy with how the team does things, this would be the point where you can try to change them.
You can suggest adding or changing code reviews, incorporating some coding standards, adding unit tests, adding automated syntax checking, leaving comments about more subjective things or whatever else.
How do I give good feedback?
Ask questions. There may be reasons why things are the way they are, so don't automatically assume you know better. This is especially true if you're more junior than them, but it also applies if you're more senior. Questioning something may also help lead the other person to the issue without you having to explicitly say it's a problem. "Would this work if X=3?" might be better than "This breaks when X=3", although in that case the difference between the two isn't too big.
Phrase things in terms of "I" whenever you're pointing out something subjective to emphasise that it's your opinion (which they can disagree with without you being wrong). So instead of "This is hard to follow", say "I find this hard to follow". In many cases you can also add "I think" or "I don't think" to "soften" a statement a bit.
Avoid "bad" words like "bad", "awful", "terrible", "hideous", etc. that don't give any information about what the actual problem is.
Point out the problem and suggest a solution. If you just point out a problem, they may not know the solution. If you just offer a solution, they may not know the problem. For example, "I find this hard to follow; you should split it into multiple methods".
"Soften" what you say. I find that criticism comes across better if you "soften" what you're saying a bit with words like "a bit", "maybe" or "might". To modify the above example, "I find this a bit hard to follow; it might be better to split it into multiple methods". This is a bit of a trade-off, as it could also make you come across as a bit more uncertain.
Avoid minor or excessive comments. I wouldn't put a hard threshold here, as necessary comments are necessary, no matter how many there are. But a review with too many comments may be overwhelming or disheartening for the reviewee. If you're leaving many comments, some of those might be related to minor or subjective things that are better left unsaid. A minor spelling or formatting mistake is probably okay to point out if there aren't too many other comments, but if you have a few dozen other comments, it might make sense to skip that one and possibly fix it yourself in that pull request or as part of a later one. Similarly, a comment presenting a different way to do something that's okay as is could potentially also be omitted if there are too many other comments already. Although you should expect to leave a few more comments than usual for new team members.
People are different. Different people may be more or less receptive to feedback. This isn't to say you should treat them differently, but rather understand that what works for a few people may not work for everyone. Try to find an agreeable consistent way to treat everyone, and accept that maybe you have to occasionally tell someone something that won't be received too well.