First time Warlock build

I did perform perfunctory due diligence and attempted a search. Nonetheless, to clarify, I see that some Invocations have prerequisites and others don't. If using an Invocation without prerequisites, must the spell being used by the Invocation be a Known Spell? Or, are those effectively "gifts" from your patron that you are able to use without actually "knowing" diddly squat about them? Are such Invocations effectively in addition to your Known Spells? Given RAW, it appears that way to me. 

If an Eldritch Invocations let you cast a spell, you can do so without knowing it.

Thank you very much!👍

Oh, and if anybody sees this, it also appears that some of those Invocations are "stackable" similar to a higher level Paladin's Smites? For example, Thirsting Blade and Eldritch Smite? Although to be at least even remotely within RAW, only one of the Thirsting Blade attacks would receive the benefits of eldritch Smite? 

Quote from pafender >>

Oh, and if anybody sees this, it also appears that some of those Invocations are "stackable" similar to a higher level Paladin's Smites? For example, Thirsting Blade and Eldritch Smite? Although to be at least even remotely within RAW, only one of the Thirsting Blade attacks would receive the benefits of eldritch Smite? 

Eldritch Smite explicitly says it can only be used once per turn. You can use it in conjunction with Thirsting Blade in the sense that you can use it on one of the two attacks Thirsting Blade allows you to make with a single Attack action, but you can't use it on both of them.

Whether invocations are "stackable" depends on the specific rules associated with each of them; there isn't a general rule about that.

Quote from pafender >>

I did perform perfunctory due diligence and attempted a search. Nonetheless, to clarify, I see that some Invocations have prerequisites and others don't. If using an Invocation without prerequisites, must the spell being used by the Invocation be a Known Spell? Or, are those effectively "gifts" from your patron that you are able to use without actually "knowing" diddly squat about them? Are such Invocations effectively in addition to your Known Spells? Given RAW, it appears that way to me. 

 AJZamo asked a similar question for a specific Invocation in Does One With Shadows (2024) require having & preparing Invisibility?

Plaguescarred already answered here, but I'd like to include this detail about casting without slots:

Quote from TarodNet >>

[...] It's as ClassicCharismaCaster said, but I wanted to add that since the spell is cast without slots, it's always cast at its lowest possible level. See What level is a spell if you cast it without a spell slot? in the SAC, and:

Casting without Slots

There are several ways to cast a spell without expending a spell slot:

Cantrips. A cantrip is cast without a spell slot.

Rituals. Certain spells have the Ritual tag in the Casting Time entry. Such a spell can be cast following the normal rules for spellcasting, or it can be cast as a Ritual. The Ritual version of a spell takes 10 minutes longer to cast than normal, but it doesn’t expend a spell slot. To cast a spell as a Ritual, a spellcaster must have it prepared.

Special Abilities. Some characters and monsters have special abilities that allow them to cast specific spells without a spell slot. This casting is usually limited in another way, such as being able to cast the spell a limited number of times per day.

Magic Items. Spell Scrolls and some other magic items contain spells that can be cast without a spell slot. The description of such an item specifies how many times a spell can be cast from it.

Also, I'd say your question is similar to Do you need to have See Invisibility prepared to use the Diviner's Third Eye See Invisibility feature?, just for a different feature.

Excellent thank you all!