I know there are some buildings that will convert food surplus into things, but without those buildings, am I just paying upkeep on buildings that are worthless now? I noticed there's one tech that gives +100% food surplus bonus. What does that mean?

Once you have all the population full you can feel free to remove +food buildings to save on upkeep. Just remember you need to have enough food to sustain the current population or they'll start starving and you'll lose population!

What is that tech that gives +100% food surplus bonus, then? Does that affect the buildings that convert food surplus?

It converts 100% of food surplus to industry. If you have a system with a food suplus and population capped you can build colonizer and send then to developing systems and scrap them transfering population to the system.

It does.

If you have Surplus Food (e.g. 50 food produce 30 eaten -> 20 Food surplus) and build the 100% Food Surplus bonus you will have 40 Food Surplus.

this either boost your growth rate (as it is based on thre Surplus food in the System) or- if you use the Food converter boost your Production further, but halt all growth in the System (so if there is still one slot left to fill up and you do build the Food->Prod Converter it will never fill up this single slot - so you have to scrap the Foodconverter and rebuild it (wich is kind aodd if you ask me...)

But in Late game you can have insane amounts of Production due to the 100%bonus Surplus food + 100% Surplus->Industry conversion.

I once ended up to turn all my Production Planets into Terran with Foodfocus as it gave me a greater overall bonus then Productionbonus and any other Maxtier(or max-1 Tier palnet type)...