UK experience here. All military units (including ships) experience staffing ‘churn’ as individuals join and leave for career and other reasons. Some military units are periodically completely reset (e.g. a new ship or a ship that has been extensive refitted), and they then have extensive training regimes to become ready for operations again. The churn then starts again. Some units persist for long periods of time (e.g. a formed air squadron) and do not get ‘reset’ under usual circumstances, although they do have churn. Others (e.g. army formations) have repeating multi-year cycles of training, being ready for ops, and other tasks.

The institutional memory is notionally preserved by the instructors in the schools and training centres (as well as documented Standard Operating Procedures, standing orders, etc). New joiners will have undergone standardized training, and although they still have to establish their role (storming, forming, norming, etc) when they join an existing team, they do in principle understand what they are supposed to be doing. In some cases, new joiners bring valuable new knowledge, skills or attitudes, so churn isn’t always bad. Individuals who have joined up for the long term will themselves have cyclic periods of training, with initial trade training followed by time in a unit, then additional specialised training at a school, then more operational tours, etc. Some highly-trained individuals may be embedded in the units themselves (e.g. at a barracks) to provide standardized, decentralised on-the-job training / supervision to newbies who have joined from a military trade school.