THT stands for through-hole technology. THT is a PCB assembly technology used less than SMT, done by component pins plug-in pre-set holes.
THT soldering has stronger mechanical stability than SMT soldering, but it can't meet the fine-pitch requirements.
THT components are usually for plug needs, such as USB, tap-C, connectors. Capacitors of large capacitance can't be surface mounted, and they are usually also THT components.
The THT components area on the circuit board is much larger than SMDs'. And THT components have long pins so that they can plug into the pre-drilled holes on the PCB.
Here is the THT assembly process:
Step 1. Let THT component pins through-hole manually
Step 2. Wave soldering to let tin fill the gaps between THT component pins and through-hole walls
Step 3. Trim THT component pins to be within 2mm
Step 4. Clean the extra flux on the PCBA surface
Step 5. (optional) IC programming, functional testing, thermal aging, end-product assembly