A CE marking cannot legally exist on its own, it has to be backed by a declaration of conformity which in turn has to be backed by appropriate technical documentation, sometimes including lab test reports. The marking alone is meaningless because it doesn’t tell which aspects (EMC, RoHS, specific product group standards) are covered by it. Claiming conformity by adding a CE mark to non-conforming product could be worse than omitting it entirely, exposing you to very substantial fines either way.
You have to check all potentially applicable laws against your product. Some laws/directives are mutually exclusive (e.g. RED vs. EMC directive), some might even prohibit a CE marking and declaration of conformity for a specific aspect (while still allowing it if mandated by other laws). Your product may be exempt from some regulations, but not all of them. Some laws may be applicable independent of any CE marking and declaration of conformity.
Usually, there are broad and explicit exemptions for anything you’re doing as a hobbyist or for development purposes; most regulations will only cover commercial operations (“placing on the market”). Importing products into the EU can still be covered by some laws, even if done for your private use without any intention of reselling.
tl;dr: do not sell any product without checking the laws, but ignore any commercially used markings for your own private hobby projects. Do NOT copy markings without understanding their meaning.