From my experience (and I have much more experience in the tenkara market than in spinning and BFS, but I suspect the situation is similar), major manufacturers like Daiwa and Shimano start planning production for the 2020 season in the summer of 2019. They will take orders then from their major dealers, for delivery in the winter and early spring. Those large orders, placed well in advance, get filled. They then open up orders for their smaller dealers in November or December. Those orders probably get filled (much of their production plan is based on the early orders, with "some" extra production). Any orders that come in after that Nov-Dec period are filled if the rods are in stock. If they are sold out, they will likely be sold out until the following year.

The smaller manufacturers I have dealt with will do a production run if you order a substantial quantity (possibly 30-50 rods), but that production run will be in 3-4 months, as their production planning is always 3 months in advance.

I have no idea how PLAT and the other merchants place their orders, but I would imagine they try to estimate demand as well as they can and order for delivery well in advance. If there is a surge in orders, they ship what they can and order more from the manufacturer. If the manufacturer is sold out, you are out of luck.