I am a noob in electrics so pardon my ignorance. What I see from an electrical system of C152 (pasted below): there are two busses, connected to each other, but alternator is fed only to the first bus. Does it mean that each component connected to the second bus takes energy from the battery and due to that battery is constatly discharging and alternator must additionally load the battery? If that is true (correct me if I'm wrong please) why don't connect alternator to both busses?
Consider them to be a single bus. There are two separate buses joined by a "bus tie" wire, either for packaging purposes if the two are together on the firewall (to shorten the linear space used) or because the second bus is mounted somewhere else (like on the back of the firewall in the cockpit) for wiring efficiency. Although not shown, there could be a fuse link in the bus tie wire between the two, to protect the primary bus and it's higher-importance services from short circuits in services running off the second bus (gotta protect that cigarette lighter!).
With everything connected together, both buses are energized by the alternator if it's running, and the battery if the alternator is off line. Whichever source's voltage is higher will be supplying the juice. The alternator runs somewhere around 28v, and the battery is somewhere around 24 ish, so when the engine is running you'll measure 28v +/- at any point of the bus, and the battery just sits there getting and staying fully charged, and when off, it drops to the battery voltage of around 24 +/-, and will decay as the battery's charge is consumed (things will start to stop working as the battery drops below around 20-22 volts).
The hydraulic analogy is limited, but is useful to mentally picture things. You have two hydraulic manifolds interconnected, and one of the manifolds is connected to a 28 psi hydraulic pump, as well as a 24 psi accumulator. When the pump is running, the manifolds will be at 28 psi, and the excess pressure being made by the pump above the accumulator's 24 psi operating pressure ensures it stays fully charged. If the pump is turned off, the system is pressurized by the 24 psi accumulator and the system pressure at any point on the manifolds drops to 24 psi and decays from that point as the accumulator is depleted.