Do "always prepared" Spells cost spell slots?
Warlock spell slots are equal to the highest level of Spells the Warlock has access to. Casting a lower level Spell will always "upcast" it to the highest level available.
Warlock 5e: DnD 5th Edition Class Guide
Warlock Spell Slots Are Separate: In BG3, Warlock Pact Magic uses its own spell slots and does not combine with other class spell slots.
Consumed Shadow Weave - bg3.wiki
From bg3.wiki. ... Level 1 Spell Slots Unlocked: 3 You gain an additional Level 1 Spell Slot. . Spells Known: 3 Choose 1 additional Spell from the Ranger Spell List.