No other coauthors, just me and my supervisors—is this normal?

I’m a PhD student and I find it a bit strange to work and publish exclusively with my supervisors, though I understand this might vary depending on the field or funding. In my field, it's not uncommon for papers to have very few authors. However, in other labs, projects including engineering especially often involve research engineers and other collaborators. My lab doesn't have any RAs and my funding doesn't come with external collaboration. I have also tried expanding my network at conferences but haven’t established any formal collaborations yet.

I guess my concerns are mainly from two perspectives: Careerwise, is it strange that I have no collaborators by any means and keep publishing only with my supervisors during my PhD? Second, on a personal level, is it normal for a PhD student to handle almost everything independently without any support from lab peers or other resources? I would love to work with research engineers who are way more experienced than me on certain aspects of the work, as well as with others on interesting shared ideas, but unfortunately I simply don't have these resources.

As @Buffy has mentioned this depend on many factors like field, project, topic, etc.

But there is more, I say that the situation of a PhD student publishing only with its supervisors is the most common one, because by default a PhD student only has supervisor(s) and is learning to publish, so to complete your PhD in many parts of the world, you just need to do research under mentoring of your supervisors, publish papers (optional in many parts of the world), and then write a thesis and graduate.

There is no (strict) need to write collaborative papers. Its the default situation most PhD students are in.

I think having collaborators is something extra to your PhD trajectory, which is nice, but not a requirement, and it varies greatly between fields.

I think this depends almost entirely on the sort of project(s) you undertake, and especially on their scale. Some projects require teams and others don't. It is also field related. In math, it is typically a single student and their advisor. In applied math it might be a team, or not. My CS students all worked alone with advice, but teamwork can also occur for some projects. In high energy physics it seems inconceivable to me (an outsider) that it isn't a large team. Outside STEM fields, students may tend to work alone, rather than in groups or teams, though there are exceptions, perhaps archaeology.

If you are making progress and publishing, then I'd guess you are in a sufficiently good place. But I'd also suggest that you find a way to build a circle of contacts with future collaboration in mind. In my field it is conferences where one builds such circles and they can wind up very productive. Perhaps you have a way to make that happen.

You can also see if it is possible to get introduced by your supervisors to those within their circle(s).

I think your concern is moreso not having collaborators rather than no co-authors. Having fewer co-authors on publications will not harm your academic career, espscially if you are pumping out several high quality pubs a year. If anything, it enhances your research team's credibility. I have seen authors from the same research groups get cited several times in the same papers especially when their work solves many gaps in their research domain. Usually its a sign of expanding the literature.

Second, on a personal level, is it normal for a PhD student to handle almost everything independently without any support from lab peers or other resources

It is certainly not uncommon. It varies depending on what institution, the nature of your work etc. However, I wouldn't consider it against the norms. As a PhD student, you are considered an expert in your field. As such, there may be certain roles you are expected to use your own auto-didactry to figure out independently. Your advisors may invested in you to learn independently because they may rely on the fact that you will be able to solve problems with minimal support. Think of hiring in certain fields, especially engineering; you want to rely on an expert in an industry or lab that an expert could figure out independently.

However, this could also very greatly. Generally, I have noticed more technical areas will have less skill training if it is not as common of an area and a generational gap in mentor-mentee relationships. Just an observation.