Decision-making is a key skill in software engineering; developers constantly make choices all throughout the software development process, from requirements to implementation. While prior work has studied how developers make decisions in various contexts, the choices that developers make while choosing what solution to write in code remain understudied. In this mixed-methods study, we examine the phenomenon where developers select one specific way to implement a behavior in code, given many potential alternatives. We call these decisions implementation design decisions. Our mixed-methods data includes 46 survey responses and 14 semi-structured interviews with professional developers about their decision types, considerations, processes, and expertise for implementation design decisions. We find that implementation design decisions, rather than being a natural outcome from higher levels of design, require constant monitoring and reconsideration of higher level design choices, such as requirements and architecture. We also show that software developers have a consistent general structure to their implementation decision-making process, although no single process is exactly the same. We discuss the implications of our findings on research, education, and practice, including insights on teaching developers how to make implementation design decisions.