I wonder how one does like queries.

After decompression, with the performance characteristics you'd expect. If it has to come off disk it's still a win or at least usually breaks even in their measurements. https://cedardb.com/blog/string_compression/#query-runtime

The paper suggests that you could rework string matching to work on the compressed data but they haven't done it.

s, jst cmprss ll qrs b rmvng vyls!