God there are so red flags here in terms of how you're being managed and on-boarded, and also unhelpful behaviours on your part. Let's begin...

These are all the things that stick out as red flags to me, in terms of your company - all based purely on what information you've provided, with speculation:
- No onboarding process
- No documentation to help someone getting started on the project
- Giving a junior dev a task nobody has tried to do before is generally a bad idea. Giving a task that is unknown to the junior, but something that a senior can cover for or at the very least assist in is sane.
- Shit code review; you fucking up twice so bad that a release gets delayed by a month means there's a systemic failure in how your code is reviewed, and QA tested, before being released. That's not just on you (because we all make mistakes), that's on your company for not making their integration and delivery more protected from mistakes like that, or being able to quickly roll them back or fix them.
- YOUR manager has been gone since December. You're a junior. Just lol, if you aren't laughing - you should be crying. The most critical person in your start at this job is missing, and very obviously not being replaced by anyone competent enough to manage you (otherwise they'd be or soon will be a manager).
- Senior telling you that they shouldn't be helping you with certain things at this point: Yeah look, I can understand what the guy is saying, you've been there for 8 months...but you weren't on-boarded properly by the sounds of it, your manager is fucking GONE, and you were giving shit tasks to learn with seemingly with no support...according to how you tell the story. 8 months is well past most probation periods. You may be sorry that you're still asking these questions, but if there was a way for you to learn it and not have to ask, that would be a lot more helpful - go with that.

Here are your unhelpful behaviours, especially as a junior:
- Not asking questions often enough and soon enough
- Not following up on questions already asked, to get answers (probably to avoid being a bother)

Really that's it - that's kinda all you gotta make sure as a junior that you absolutely do not fuck up, is ask questions like your life depends on it. I bet a lot of your slowdowns are due to you trying to bust trough the wall with your head with certain issues, rather than asking, because now you've stigmatised yourself about asking questions given ""how long"" you've been there for, and what that one guy said.

Please make sure you are asking questions AFTER you tried to figure it out yourself - maybe it's about how you've been asking questions, you know? There's nothing more annoying than someone asking you a question and just expecting a full answer handed to them on a silver platter, especially if its about something that's involved and would take time working out yourself, or something that is known to be able to be easily looked up. That being said - look up your own answers, and try your own solutions. Time box them to 5, 10, 15, 30 minutes...depending on how big the thing you're working on is, and then ask the question...telling the person what you've tried, where you've looked, and what you think the answer might be. Show that you put some effort into it...but don't do what you HAVE been doing, waiting too long to ask!

>I know I should just focus on improvement, but I am not sure how

The answer to that questions has been missing, apparently since December. Seriously - this is what the fuck your manager is supposed to help you figure out. If you do get performance managed, that may be a blessing in disguise because it would mean knowing where you are lacking and forming a plan to bridge that gap and then review it after a certain amount of time. Don't view the PIP as some dreaded thing -- it is meant to make you productive and effective. Yeah it sucks in essence that "it got that bad", but given the circumstances BESIDE you, it IS that bad.

>Should I be writing reminders to myself to always double check everything?

Writing down your own version of whatever process you need to do when you're submitting a change, or doing whatever at your company would be super helpful...ESPECIALLY if they don't have any such documentation that everyone can read already (which should be a fucking crime).

>Should I write down the steps to every process?

Every process sure, but in varying detail. Some things you may want to write down step by step. Some things you may just want to write down the most important thing to remember/check. It all depends. Either way...write down whatever you KNOW you will not retain in memory AND that cannot be easily accessed online or through your company's documents and documentation.

>Its hard for me to know whats a junior engineer error and whats an error I shouldn't be making at all.

You're a junior. You're meant to be making all of the errors - but you aren't meant to be allowed for those errors to proceed to consequences. Thats literally what the work processes and all the fucking DevOps and systems infrastructure is meant to be there for...to stop errors, both machine and human, from making it into production. Writing shit code is on you...but letting shit code into production is on whoever created (or didnt safeguard from) a system that lets that actually be deployed...so...the team, the rest of the company.

Writing shit code and having your team review it and the company's pipelines alerting you to errors is how you learn that it was indeed shit code, and then, through the grace of professionalism, are shown the right way, or at least given the opportunity to figure out how to fix it.

I don't know just how "bad" you may actually be (if I was to just believe you - pretty terrible; that's how you feel about you). I am only reading your "take" on it...no idea what your team actually thinks. Maybe you're overreacting, or maybe they are just incompetent assholes...hell, maybe its both.

It doesn't matter. What you gotta do, is start asking questions (more often, sooner), start writing down what you know (so you don't forget, and avoid fucking it up), and talk to someone about these very worries you're having. Is someone replacing your manager currently? Talk to that person. If nobody is, find out who your managers manager is and raise these concerns with them.

Caring about the juniors you hire is on the company. You aren't being hired to be cannon fodder. You're an investment: they get someone who is fresh and can be moulded (on the cheap, relatively), knowing they wont be productive for a good while, but will be able to pick up a lot of the processes specific to that company without "old ways" getting in the way.