The simplest answer is that they are slow growing varieties of their "normal-sized" relatives. Due to their slow growth, they remain at a small size for a long time.
An Eastern White Pine (Pinus strobus) might reach 50-75 feet in 40 years.
This dwarf Eastern White Pine (Pinus strobus 'Macopin') might reach 8' over the same time period.
There are varieties of Dwarf Conifers suitable for almost any growing area in the U.S. Some prefer shade, some prefer plenty of light. Some can withstand very harsh winters. Others are happier in more temperate regions. The layout of your landscape also has an effect - if you live in the mountains but have a nice, warm sunny south-facing slope, you might be able to plant varieties more commonly found in the lowlands.
Before planting, mix some compost, composed manure, or bark mulch in with the replacement soil to equal about 1/3 of the volume. The diameter of the hole should be 6" - 12" wider than the root ball or container of your tree, and deep enough for the entire root ball to fit just above (about 1") ground level. Place your new tree next to the hole, lying on its side. Then carefully roll it into the hole. Plant so that the root ball is an inch above ground level. Replace the soil until your tree is completely stable and the hole is filled in. Tamp down the soil with your foot. Water it in very well. Apply some mulch to help retain moisture and protect the valuable roots.
Dwarf Conifers are extremely easy to maintain - that's part of what makes them such perfect landscape plants. They naturally grow into the desired shape you bought it for - without pruning or shearing! All they need is an adequate amount of water and only for the first 2 growing seasons or the first 2 summers.
Dwarf Conifers do just fine without the addition of fertilizers or other special treatment if planted properly. If you are doing new planting in spring or summer, you may want to mix some 18-24-6 fertilizer to the planting soil to give the tree a good head start. For new plantings in fall or winter, wait until spring to fertilize.
The use of fertilizer on dwarf or miniature plants can affect the overall growth rate and change the desired growth habit. Fertilizing for plant health is a good practice, while adding fertilizer to increase growth rates is not recommended for dwarf conifers.
We grow our plants so that they are healthy, hardy and disease and insect free when you receive them. A healthy tree generally will not have problems with disease or insects. The best prevention you can do is to make sure they have enough water and the appropriate amount of sunlight.
As with all landscape plants, trees, shrubs, and garden perennials, it is very important to keep them watered and look-after for their first year. During this critical time period, the plants are developing new roots and doing their best to extend their roots into the surrounding soil. Until they accomplish this task, which takes a year, they are vulnerable to drying out. Once a root has completely dried out, it will die. Trees need 1" of water per week. In general, water your trees from ground thaw through October. If you are in a location where the ground never freezes, water all year round (if rain is not sufficient). We do recommend a good quality rain gauge to insure proper watering through out all season, it gets dry in the winter also.
One of the best sources for information and answers on any plant or tree question is your local Agricultural Extension Agent. Your local agent is knowledgeable in a broad range of planting subjects, and will be familiar with any issues related to your particular location. https://www.ces.ncsu.edu/