Really, only UPS that have value are online ones. They take AC and convert it to DC at around 48-96V. Batteries (usually 12V ones connected in chain to get that 48-96V) give out same voltage. Batteries and output of AC-DC converter connected together through diodes. Regardless of what source is giving power, that 48-96V DC then converted to 155 or 315V DC depending on grid voltage, 110V AC or 220V AC. This 155/315V DC then converted to 110/220V AC sine. There are no direct connection between input and output and switching to battery and back occur absolutely seamless. When mains disappear, current continue to flow from batteries like nothing happened. When power is back, you get current from AC-DC converter again. Last stage in that scheme, DC-AC converter, is really not necessary for most tech usually connected via UPS.

You could find parts of online UPS as separate modules, also today you could find modules for building solar power systems based on online UPS principle, when in addition to mains AC-DC converter and battery, to the same 48-96V DC rails also connected solar panels that will be used when they give enough voltage in absense of grid power. Or they will charge batteries if grid is OK. So you could build DIY cheap online UPS using this modules, car batteries and whatever additional source of electricity you have and get a power source that will seamlessly move from one source to another and back by design. If you need it only for modern devices with switching power supply, you don't need to buy final DC-AC converter, which is usually not something cheap, due to expensive high-voltage high-power transistors, you could power your tech from 155/315V DC without any issues.

Really, only UPS that have value are online ones. They take AC and convert it to DC at around 48-96V. Batteries (usually 12V ones connected in chain to get that 48-96V) give out same voltage. Batteries and output of AC-DC converter connected together through diodes. Regardless of what source is giving power, that 48-96V DC then converted to 155 or 315V DC depending on grid voltage, 110V AC or 220V AC. This 155/315V DC then converted to 110/220V AC sine. There are no direct connection between input and output and switching to battery and back occur absolutely seamless. When mains disappear, current continue to flow from batteries like nothing happened. When power is back, you get current from AC-DC converter again. Last stage in that scheme, DC-AC converter, is really not necessary for most tech usually connected via UPS.

You could find parts of online UPS as separate modules, also today you could find modules for building solar power systems based on online UPS principle, when in addition to AC-DC converter and battery, to the same DC rails also connected solar panels. So you could build DIY cheap online UPS using this modules, car batteries and whatever source of electricity you have. If you need it only for modern devices with switching power supply, you don't need to buy final DC-AC converter, which is usually not something cheap, you could power your tech from 155/315V DC without any issues.