Seize the opportunity to build it yourself
Depending on your actual company/situation, this might not work. It might not even work in most situations. But it did in mine, with spectacular results for my career, so consider it as an option.
You've been handed a brand new department with no systems, processes, documentation, existing employees or any other kind of infrastructure. You need to get it up and operating immediately because there is ongoing work that can't be delayed.
And therein lies the opportunity.
This department needs to be built. In most companies that would be done by senior, experienced people who've done similar work beforehand. Instead, they've got you.
If you want to become that senior engineer yourself one day, this is your chance to gain invaluable experience and achieve results that will fast-forward your career five years.
In my case, the previous employee left. I was handed a spreadsheet, an inbox, some largely useless documentation, and asked to take over the whole thing. So I did.
Don't have a process? Invent one.
No QA? Create your own QA checklist. No idea how to do that? Do some googling. Copy somebody else's and get going. Update it as you go.
No tooling? Build your own. Need to pay for a 3rd party tool? Find a free version that does 90% of what you need and work with that. Or make a case for it to your boss. If they push back compare to other departments in your company and what they typically have to spend on tooling to justify why it's standard and necessary.
If it's cheap, consider just paying for it yourself. Yes, it's a bad precedent to set. Yes, it's not something you should ever have to resort to in a functional company. But if £50 a month is what's standing between you and career advancement then just pay the £50, produce awesome results, and work on persuading your company to pay for it later.
Set up your own development environment. Either your company has a standard system they use for it, in which case ask the relevant people/department to set it up/get you a license. Or it doesn't, in which pick a cheap and/or free version off the internet that looks like it might work and go with that.
Need a senior engineer's / architect's insight? Go find some on the internet and buy their books / read their blogs / ask them questions on stack exchange / invite them to chat.
Try things out. Iterate as you go. Do whatever you have to do to get results. Don't worry about building for the long term yet. Your job is to build something that works first. Quick, dirty, hacked together, whatever. Once you've got something that works, then work on incrementally improving it.
You have no idea what you're doing so don't spend time on anything that not's going to produce immediate, useful benefits. If that refactoring you want to do won't pay for itself until 3 months down the line then forget it. Your understanding of the requirements, the options, and what you don't know you don't know will have changed so much by then that you'll want to rip it all out and start over. Which you will have to do at least a couple of times.
This is your opportunity to learn and advance without restriction, limited only by how hard you work and how fast you can teach yourself what you need to know to produce useful results. With nobody there to guide you, yes. But also nobody there to stop you.
In my case I worked myself almost to the point of burnout, which I do not recommend, but I also built that entire department starting form scratch with zero relevant experience in the space of a single year. At which point we hired an analyst to take over all the work that was taking up my time, because it was needed to do the same work on other, more important parts of the company.
Even if it isn't appreciated by your boss/other people where you work, it will be attractive experience for anyone else who might want to hire you. And that attitude of doing what needs to be done and advancing as far and as fast as you're capable of will take you wherever you want to go in your career.