While some projects within the compiler toolchain reinvent the wheel to varying degrees of increased performance/functionality (e.g. MIR, probably some parsing tools too, gold linker, lldb, etc.), the majority of projects these days take advantage of existing tooling to applying frontends and backends in different ways. Want a JavaScript to LLVM compiler? Map the JavaScript AST using an existing JavaScript parser to the LLVM AST and have LLVM generate code. The combinations are infinite and high quality. Most people (myself included) only experiment at the highest level.

All this is to say the barrier to entry in PL/compiler/interpreter implementation is only as high as you want it to be. You can do the whole thing yourself as a learning experience or you can use existing tools to finish a fairly-fully-featured project in a few hours over a few weeks.