From Find Familiar (PHB p. 294)
the familiar has the
statistics of the chosen form, though it is a celestial, fey,
or fiend (your choice) instead of a beast.
They have the same Intelligence (and Wisdom, and Strength etc.) as the chosen form.
The spell is explicit about what they can and cannot do. Specifically:
- It acts independently
- It obeys your commands
- It cannot attack
- You can communicate with it telepathically up to 100 feet - it does not say that it can communicate with you in this way
- You can use an action to "see through your familiar’s eyes and hear what it hears"
- You can use an action to dismiss/recall it
- You can use it to cast a "touch" spell
Of these the only one where lack of intelligence would be an impediment would be obeying commands. The forms mentioned all have an Intelligence of 1, 2 or 3, so:
- a command "Design the Sydney Opera House" is going beyond their capability,
- "Go there", "Come here", "Stay" are not,
- "Go through that door and scout the next room" is a DM call. The geniuses of the group (cat, octopus) might manage this; the others would probably struggle. If you want to use a familiar as a scout you would be better served "see[ing] through your familiar’s eyes and hear[ing] what it hears" and issuing short easily followed commands on a round-by-round basis.