Most commercial potting soil mixes contain little or no mineral soil. They are mostly various organic materials. These materials break down over time. As this happens, the soil level goes down in your pots.
Yes, you can add fresh potting mix to the top. Just be careful not to bury the plant tops, which might then rot. It may be best to top off the containers early in Spring, before new growth starts. Depending on what you're growing, just be careful about burying enough of the plant to cause rot.
Because the soil in the bottom of the pot is where the roots are, and that is the more depleted soil, the plants would probably benefit from regular addition of small amounts of complete fertilizer. Again, this depends on what you're growing. Some plants need little or none, some can take lots.
Even with this attention, the plants will eventually start declining. This is generally from root congestion. I've also had tree roots grow up through the pot's drainage hole, choking out my flowers. This, however, is likely to take many years.
I then recruit some help to empty the pots and start over.
They just don't stay on autopilot forever, at least for me!
