The Foundational Skills Toolkits have changed with ARC Core over the years. They were first released prior to ARC Core in 2016. Originally they were:
Not part of ARC Core (per 2021 website of what was included with ARC Core).
Not a part of Tier 1 instruction (not for primary instruction).
Per ARC, they were "designed to provide intensive word work for small groups of students who have demonstrated a need for specific decoding skills in reading levels K–2.
The Foundational Skills Toolkit provides mini-lessons and instructional strategies to help teachers implement code-focused instruction. Students receive explicit instruction in letter sounds, phonological awareness skills including rhyming, segmenting and blending, sight word development, and phonics including letter-sound correspondences, blends, and digraphs, and eventually decoding."
Take a look some of the contents of the original Foundational Skills Toolkit.