Check if B is composed of substrings of A of length n
Excel, 114 bytes
A nice low bar to start with.
=LET(x,SEQUENCE(LEN(C1)),t,TRANSPOSE(x),AND(MMULT((x&lt;t+A1)*(x&gt;=t),IFERROR(FIND(MID(C1&amp;REPT(&quot; &quot;,A1),x,A1),B1),0))))