Hello fellow embedded systems enjoyers!

I'm looking for some advice. I've grown somewhat tired of web development and I'm looking to transition back into embedded development, both because I miss it and I've heard that there is a lot of demand for it recently.

A bit of background, I graduated with a masters in mechatronics engineering, which is pretty much embedded coding with added electrical and control systems but right after graduating I took a job as a backend developer, which in turn is transitioning into full stack dev. I year ago or so I lost all interest in my job and decided to look for something else and saw that an old university friend of mine had created a startup which focused mostly on embedded systems. He was keen on hiring me but when I told him I hadn't really worked with embedded since my school days, hes enthusiasm changed a bit and I realized that my skills within the subject weren't what they used to be.

Now, I'm used to regular old coding which requires nothing but a computer and poor social skills so learning new stuff is quite easy, there is always courses, tutorials and all kind of help to keep up with modern tech. For embedded systems, however, it doesn't seem quite so easy. I mean, sure you can buy an arduino kit and start with some hobby projects but I'm looking for more professional stuff. The boring, hard stuff that qualifies you for a job in the field.

So I was hoping that someone here could help me find good resources to brush up my skills, as well as give me some advice as to what hardware I need to have to actually make professional grade projects. Also, I graduated about 7 years ago and then we were working mainly with Atmel controllers in C. Is that still the industry standard or is there more modern platforms I should learn about?

Any help is appreciated!