The circle spells are, as you say, always prepared on top of your other spells.
There are a number of spells you have prepared from the basic Spellcasting trait common to all druids:
You prepare the list of druid spells that are available for you to cast, choosing from the druid spell list. When you do so, choose a
number of druid spells equal to your Wisdom modifier + your druid
level (minimum of one spell)
If you are a land druid, then once you reach certain levels you have other spells that are always prepared on top of what you have chosen (note the bolded section).
Once you gain access to a circle spell, you always have it prepared,
and it doesn’t count against the number of spells you can prepare each
Remember that druids can choose (change) their "normally" prepared spells every day. So if you already have a circle spell prepared as one of your normal spells, then you can simply prepare something else because being a land druid will mean you have it always prepared anyway.
E.g. If you are an arctic druid, then no need to prepare Spike Growth as one of your standard prepared spells, because as an arctic druid that spell is always prepared anyway on top of your other spells.
Spells known and Spell slots
None of the above affects spell slots, which is the number of spells you cast per day.
You have spells known/prepared and you have spell slots.
- Spells prepared are the spells you have available and ready.
E.g. a 3rd level druid may have, say, 6 spells prepared which can be 1st level or 2nd level spells. A 3rd-level druid can only choose 1st or 2nd level spells but you could prepare, say, four 1st level spells and two 2nd level spells, or 3 1st level spells and 3 2nd level spells etc.
- The spell slots that you have are indicated on the class features table for druids. For a 6th level druid that would be four 1st level slots and two 2nd level slots. This is how many spells you can actually cast in a day.
When you cast a spell, you can pick any spell that you have prepared and that will use up a spell slot of at least the level of the spell (some spells have greater effect if you spend a higher level slot).
E.g. If you cast "Faerie Fire" - a 1st level spell - you must spend a 1st level spell slot or higher. That slot is now "gone" until you take a long rest or use some other ability that gets slots back.
Circle spells are additional spells prepared and available but do not affect number of slots. But note that land druids do get the "Natural Recovery" feature that allows them to recover some spent spell slots during a short rest, allowing you to cast more spells in a day.