I'm not really sure what you are talking about, you need to synchronize both the phase of the power and the frequency. If they differ even slightly you damage your equipment. This is why when running multiple inverters in parallel they require both a Communication cable and a power sharing Cable in order to ensure that they stay perfectly synchronized. Inverters capable of doing grid tie have equipment in them to monitor the mains power and make sure that they stay synchronized with it. Whereas an off-grid inverter does not contain that equipment which is why they are generally cheaper

There are several different possible phases for 120 volt they are not all the same. And while hooking to the mains may be standard practice for something that can grid tie it is not standard practice for an off-grid inverter. You are specifically expected to have a secondary panel specifically for that inverter. The mains power will reach that secondary panel through the inverter when it's in pass through but the panel should in no way be connected to the mains directly (other than a ground bond between the secondary panel and the mains)

With that having been said, I have no doubt that solar installers have instead opted to continue to use inverters that are capable of grid tie even in a installation that will not be backfeeding. Probably just to reduce the amount of inventory required so that they can get full purchase orders. But that doesn't change the fact that a proper true off-grid inverter is not capable of tying into the mains without destroying itself

The fact that you say you were a solar installer, but you're not even aware of something as simple as phases and frequency synchronization is part of the reason I feel like solar installers are wildly overpaid. You're basically doing the bare minimum without really understanding what it is you're doing