Dryers with condenser coils are the most popular. The tumble drums are heated by blowing air over an electric coil. Heat and moisture are emitted into a container as vapour and condensed into water. Condenser dryers are closed systems. The air passes over a heating coil and into the drum. The air is expelled into a condenser chamber, where water is extracted, and the dry air is heated and returned. Collecting water needs to be emptied regularly.
It is fast, requires a water reservoir to be emptied, weighs the clothes before they are dried, and starts on a delayed timer to prevent over-drying or using low-price electricity. These are better and stretching to this level will get you the best heater/dryer.
Condenser dryers without heating coils are heat pumps. The mini-air conditioner/fridge is a closed-system model. When a compressor compresses refrigerant gas, hot air is extracted and blown into the tumble drum during the process of blowing air through the coils. From the coils, water is collected. That's why it's cold inside the fridge and hot outside.
Generally speaking, they tend to be the quietest and gentlest on textiles because of their lower temperatures. Their size is only available in medium (7-9kg) or large (10kg or more), and they cannot be wall-mounted. Upside: low energy use. Said to be gentler on clothes. Downside: Long time to dry. Cannot be wall-mounted. Need to empty a water reservoir. A lot of tech goes wrong.