You cannot hide anything on the internet anymore, the full IPv4 range is scanned regularly by multiple entities. If you open a port on a public IP it will get found.
If it's a obscure non-standard port it might take longer, but if it's on any of the standard ports it will get probed very quickly and included tools like shodan.io
The reason why I'm repeating this, is that not everyone knows this. People still (albeit less) put up elastic and mongodb instances with no authentication on public IP's.
The second thing which isn't well known is the Certificate Transparency logs. This is the reason why you can't (without a wildcard cert) hide any HTTPS service. When you ask Let's Encrypt (or any CA actually) to generate veryobscure.domain.tld they will send that to the Certificate Transparency logs. You can find every certificate which was minted for a domain on a tool like https://crt.sh
There are many tools like subdomain.center, https://hackertarget.com/find-dns-host-records/ comes to mind. The most impressive one I've seen, which found more much more than expected, is Detectify (which is a paid service, no affiliation), they seem to combine the passive data collection (like subdomain.center) with active brute to find even more subdomains.
I'm just a hobbyist, but here's some resources that have helped me. My interests are specifically for musical applications, so that might affect some things.
"The Scientist and Engineer's Guide to
Digital Signal Processing" is free online. This has been my most used text so far. [0]
Introduction To Digital Filters [1]. Another book free online. I haven't dug very deep into this one yet, but select chapters have been helpful so far. Juluis O. Smith III has several other books on the topic that seem useful too.
Earlevel's Wavetable oscillator tutorial. Create a mip-mapped wavetable oscillator in C++. A really nice balance of theory and practicality in both the explanations and code/ [2]
The music-dsp mailing list and the KVR DSP forum [3] have both been extremely helpful. I'd seriously recommend lurking the KVR forum for a bit if you're interested in musical applications of dsp. It's a bit like HN in that you'll ask "how do I replicate this 80's digital synth" and sometimes the original programmer will respond.
Aside from that, blogs specific to your interests. Plugin manufacturers can often get pretty technical on their blogs. I recommend reading through the archive of the valhalladsp blog if reverbs interest you.
And not DSP specific, but working on a fun project is always a good motivator. I started off with VSTs using I-Plug/WDL-OL. Now I'm using a Hoxton OWL (guitar pedal with a Cortex M4f) and have been much more productive than ever, just because the form factor and hardware are so fun.
There's lots of repositories of code examples. AudioTK has a lot of examples and is high quality, I feel [4]. MusicDSP has a TON of code examples, though not all of them are good quality or well optimized, it's like the stack-overflow copy-pasting of DSP, but still useful for the depth. [5] There's probably a hundred others, many optimized for specific hardware, but these are the 2 that come to my mind first.
Edit: Oh, there's also tons of MOOCs and online lectures about this as well. I won't offer any advice there though, as I don't learn well through those.
If it's a obscure non-standard port it might take longer, but if it's on any of the standard ports it will get probed very quickly and included tools like shodan.io
The reason why I'm repeating this, is that not everyone knows this. People still (albeit less) put up elastic and mongodb instances with no authentication on public IP's.
The second thing which isn't well known is the Certificate Transparency logs. This is the reason why you can't (without a wildcard cert) hide any HTTPS service. When you ask Let's Encrypt (or any CA actually) to generate veryobscure.domain.tld they will send that to the Certificate Transparency logs. You can find every certificate which was minted for a domain on a tool like https://crt.sh
There are many tools like subdomain.center, https://hackertarget.com/find-dns-host-records/ comes to mind. The most impressive one I've seen, which found more much more than expected, is Detectify (which is a paid service, no affiliation), they seem to combine the passive data collection (like subdomain.center) with active brute to find even more subdomains.
But you can probably get 95% there by using CT and a brute-force tool like https://github.com/aboul3la/Sublist3r