I have read the manual and posts on steam, but some people say it done automatically, some say you just select or switch and others say you have to set your reinforcement strategy
Both are right
The source of the "reinforcing" can be either another unit (usually they are designated as conscripts) or the broken squads of the same platoon. In this case, the "reinforcing" is done automatically.
Or the second option - you can strengthen part of the platoons, if you have units in reserve. This is done by the player himself, choosing a reinforcement strategy.
The reinforcement strategy seems only available to company commanders and even then manpower is always grey.
Undoubtedly, the success of a strategy depends on the availability of resources (reserves). If they are not available, then what can be the replenishment? Soldiers will not take from the air.
Also what does the colour code of green, blue, grey and red signify under 'Personnel' information. It does not seem to conform to strength.
Green - excellent condition
Blue - reinforced
Grey - not available
Red - bad condition