Context: I was part of a company acquired by FB in 2019, worked there until Dec 2022.
It really depends, based on how independent the acquired company needs to be, and how useful it would be to have overlap with code components & engineering resources with the main monorepo.
In our case, up until Dec 2022, parts our pre-acquisition monorepo were still separate, but gradually components and workflows, such as tests and reviews, were moved into the main repo. From my awareness, we didn't even start merging into the main monorepo until more than a year after we were acquired, though of course there were exploratory efforts before.
In general, I'm pro-monorepo, it makes sense to be able to update multiple interconnected components in lockstep. For the startup, we were still in research mode, so it was less urgent to spend eng/sci/TPM to incorporate with anything on the FB monorepo side...until it was.
In our case, up until Dec 2022, parts our pre-acquisition monorepo were still separate, but gradually components and workflows, such as tests and reviews, were moved into the main repo. From my awareness, we didn't even start merging into the main monorepo until more than a year after we were acquired, though of course there were exploratory efforts before.
In general, I'm pro-monorepo, it makes sense to be able to update multiple interconnected components in lockstep. For the startup, we were still in research mode, so it was less urgent to spend eng/sci/TPM to incorporate with anything on the FB monorepo side...until it was.