Hello and welcome,
This may be true but is not necessarily - the lockscreen could be independent.
But anyways yes, this is more or less the case.
For example by default on Plasma ..
You would first be greeted, after cold boot, with the display manager plasma-login-manager (or sddm if it is an older install and you have not swapped it out).
Then, after sleep or lock, you would be interacting with the lockscreen kscreenlocker.
No, because they are different softwares performing different functions.
In case it may be illustrative here is the ArchWiki on Display Managers or "DM"s;