[prhn]

What are the modern, accepted software principle, architectural or design patterns, and security best practices?

What is the best source on these topics?

I’m not being cheeky here. I’m genuinely interested, because I see these terms thrown around a lot without reference.

[tjpnz]

Not everything has to be "modern" (a term which also gets thrown around a lot without reference) to be useful.

With regards to design patterns I would recommend the Gang of Four book on the subject. It's old (to the point where it makes me feel young) but still relevant as most of the patterns you'll see in the wild will be described there or be slight variations.

For security I've heard people describe the OWASP Top Ten as the software equivalent of the Ten Commandments. It's been around for almost two decades and is also still very much relevant today, perhaps even more so going by the comment you replied to.