The more fossils you find at a location, the more you can fine-tune the relative age of this layer versus that layer.Of course, this only works for rocks that contain abundant fossils.
Paleontologists have examined layered sequences of fossil-bearing rocks all over the world, and noted where in those sequences certain fossils appear and disappear.
When you find the same fossils in rocks far away, you know that the sediments those rocks must have been laid down at the same time.
The science of paleontology, and its use for relative age dating, was well-established before the science of isotopic age-dating was developed.
Nowadays, age-dating of rocks has established pretty precise numbers for the absolute ages of the boundaries between fossil assemblages, but there's still uncertainty in those numbers, even for Earth.
The chronostratigraphic scale is an agreed convention, whereas its calibration to linear time is a matter for discovery or estimation. We can all agree (to the extent that scientists agree on anything) to the fossil-derived scale, but its correspondence to numbers is a "calibration" process, and we must either make new discoveries to improve that calibration, or estimate as best we can based on the data we have already.
To show you how this calibration changes with time, here's a graphic developed from the previous version of Fossils give us this global chronostratigraphic time scale on Earth.
Venus, Io, Europa, Titan, and Triton have a similar problem.
On almost all the other solid-surfaced planets in the solar system, impact craters are everywhere. We use craters to establish relative age dates in two ways.
A few days ago, I wrote a post about the basins of the Moon -- a result of a trip down a rabbit hole of book research.