By far the best way to learn arithmetic facts is to ‘naturally’ use them in service of solving more interesting or relevant problems. Someone who spends the same amount of time doing nontrivial word problems, pattern-discovery projects, playing a game or solving a puzzle involving embedded arithmetic, or just talking about numbers in a group will come out vastly better prepared both for recalling or figuring out arithmetic solutions per se and for mathematical fluency in general than someone who does narrow practice drills. Arithmetic drills are not only a total motivation killer for most people, but also just suck at aiding retention. Time spent on arithmetic drills in school is somewhere between a waste of time and an actively harmful punishment.

If anyone wants some primary school appropriate word problems, let me recommend the collections by Lenchner, e.g. https://archive.org/details/mathematicalolym0000lenc

Also try Kordemsky’s Moscow Puzzles https://archive.org/details/boris-a.-kordemsky-the-moscow-pu...