It’s one of those “not an exact science” things. Most of them pretty much go to the higher voltage, holds it there for a predefined amount of time, and then drop to the lower voltage. And that is that, nothing more fancy is done at all.
Victron inverters use a kind of adaptive charging, where it times how long it took to get to the absorption voltage, and then the amount it stays there is proportional to that (I think it is 5 times the time it took to get there). The idea is that if the bulk stage is really short, then the battery is probably already quite full, and absorbing too long is detrimental to the battery. I am not aware of any other inverter/chargers that use this method. Note also that this method is for LEAD ACID batteries. Not for Lithium.
If the battery voltage drops below a certain point (Something like 1.2V below the float voltage for lead acid, slightly less for lithium), the inverter/charger triggers “rebulk”, which means it goes back into bulk, absorb, and then float. The time it stays in absorb is again dependent on how long it took to get there. Again, not sure how others do it. Everyone has their own little thing that they do.
Then, Victron inverter/chargers also do a periodic absorption cycle, configurable. It won’t stay in float forever (although many others do exactly that… and it is fine for most cases).
Victron inverter/chargers also have a “storage” state. After 24 hours of floating, with no discharges, it goes to a lower non-gassing voltage (again, this is for lead acid). And in the same manner, an absorption cycle triggers every few days.
As I said, not an exact science. Everyone does it a little differently. Most simply do bulk → Absorb → Float, with a hardcoded time for absorb.
Lithium batteries don’t really need absorb, but it can be beneficial to have a similar two-step process. The higher voltage is high enough so that the balancer can operate well (ie, it should be high enough to push cells above 3.45V, possibly even 3.5V). After some time, it then drops to the lower voltage, where your cells level out around 3.4V to 3.45V, which is better for the longevity of the battery, and also switches off the balancer. For today.