Is Bitcoin completely Open-Source?
Of course it depends on what you mean by Bitcoin and what you mean by Open-Source
Most definitions of "Open-Source" focus exclusively on Open SOurce Software. There are good in-depth and rigorous definitions for what makes a piece of software "Open-Source" or not.
A few definitions of "Open-Source" allow the phrase to apply to things other than computer software. For example techopedia:
Open source revolves around the concept of freely sharing technological information so that it may be improved through multiple insights and viewpoints. Since the technology is open source, the amount of work that needs to be done is reduced because multiple contributions are added by many individuals. This concept existed way before the age of computers and even before the industrial age when people shared and improved recipes for food and medicine, for example.
The broader Bitcoin ecosystem certainly contains important concepts, ideas, services and products that are completely proprietary or contain proprietary elements. For example, some prominent ASIC mining hardware designs, some hosted wallet services.
It is arguable what Bitcoin is or is not. One reasonable definition is that is a published set of rules and network protocols by which a distributed peer-to-peer currency can be sustained.
In my view it would be incorrect to say that Bitcoin is solely or fundamentally a piece of software (open source or not).
Some people have created and maintain open-source software (e.g. "Bitcoin core") which implements those rules (or a coherent subset of them) but there can still be proprietary software doing the same job and interoperating with installations of open-source software.
The rules are managed through a process which is fairly open (see BIPs) - but ultimately which Bitcoin rules are adopted is determined only by the consensus of people operating Bitcoin nodes. It wouldn't be right to label this process as "open source".