Drain assembly with no nut underneath?

It is very common not to see a nut — it may be hidden, corroded, or the assembly works differently than you expect. Here is what to check and how to tackle it:
First — check what you actually have
The nut is there but hidden
Many assemblies have a large locking nut screwed onto a threaded tailpiece that extends up from underneath. Over time, corrosion, paint, or thick silicone can completely hide it, making it look like part of the pipe or sink base. Feel carefully with your fingers — you may feel the hexagonal edges beneath the grime.
Clean away debris and apply penetrating oil such as WD-40 around the base where the threads would be. Leave it for 15–20 minutes, then use a basin wrench or adjustable spanner to turn it counter-clockwise
It is a different type of assembly
Some modern or older designs twist apart from above — the entire flange unscrews counter-clockwise, with a locking collar that engages internally rather than a visible nut underneath
Push-button or captive designs release differently — check the top for a small grub screw or cap that twists off
If it is a sink with an integral pedestal, the nut may be tucked right up inside the pedestal — you may need a long-reach basin wrench to reach it
If the nut is completely corroded or seized
If the metal has corroded away completely, the assembly may be effectively fused to the sink. In this situation:
Soak thoroughly with penetrating oil and gently tap around the joint with a small hammer to break the corrosion bond
If that fails, use a junior hacksaw with a thin blade to carefully cut through the threaded section from underneath — make sure you protect the sink surface from scratches
Alternatively, cut the tailpiece just below the sink and replace the entire drain body as a unit
Step-by-step approach
Place a bucket and towel underneath to catch any water
Clean the area thoroughly so you can see clearly
Look for:
A hexagonal nut right up against the sink base — may be buried in grime
A threaded section that is hidden but still intact
Any grub screw or twist-lock mechanism
If you still cannot locate a nut, try gripping the flange from above with a pair of pliers wrapped in cloth, and turn it counter-clockwise — some units unscrew entirely
If it will not move, apply penetrating oil and leave it overnight before trying again
Summary
The nut is almost certainly there but is either hidden by corrosion, tucked out of sight, or the assembly is designed to unscrew from above. If the metal has corroded beyond recognition, cutting the tailpiece is the reliable way forward and will not damage the sink if done carefully.

It may have been glued on if there's no nut. Usually any basin or bath waste would have a nut underneath

If your corroded drain assembly has no nut underneath, it's likely stuck due to rust/calcium buildup, meaning you'll need pliers (channel locks), penetrating oil, or possibly a hacksaw to cut it off, as it's often fused to the sink tailpiece; first remove the stopper linkage, then grip and twist the large locknut (or basket strainer body) from below, using vinegar to loosen mineral deposits if needed.