I'm not really sure what it costs these days - i know certain projects not entirely free like they were a few years ago, but there's a pretty good "build your own redis" among other things.

It has little step-by-step tasks with automated tests. There are some other good ones like git and docker. It's pretty cool.

https://app.codecrafters.io/courses/redis/overview

It's easy to write a KV store, even a KV store with key expiration. But Redis is quite a bit more than that. Consider sharding and replication at least. Then lists, sets, zsets, bitfields, streams, geospatial indexes, hyperloglog / bloom filters, time series.

Something like LevelDB is relatively easy to write. Then you can build the rest of Redis on top of it.

Sounds pretty interesting.