Either "Replacement" or "Great Replacement" is the posited explanation for a set of observed facts, and conjectured facts (counterfactuals).
For example observed facts encompassed by the (plain) theory:
a. Visibly changing demographics particularly observable in some small towns, parts of cities, the staff of hospitals, nursing homes, and a raft of lower paid places of employment that pay insufficiently for those who had their expectations raised during the so called "celtic tiger".
b. Changing policies of immigration, driven by factors including the ponzi element in our economy , and particularly by the problem of the reduction in the ratio between the population in working-age group 15-64 years and the population 65 years or older, or the potential support ratio (PSR). I.e. Not enough working-age population to support the needs of the older retired population.
c. Photos on social media of refugees traveling to Europe, fleeing war, and so on.
So all these kinds of facts, many of which are observed through the lens of social media, and also the many counterfactuals also driven through social media, encompassing hypotheticals, and wished for scenarios, and idealised pasts, and futures, demand some kind of encompassing explanation.
So you have "replacement" which suggests a dynamic that puts people, particularly proponents of the theory, in a central relationship to these facts and counterfactuals, the theory includes them, in fact it positions them as the victim of these observed and non observed facts.
Now you also have "great replacement", which is a variation on the theory - the addition of "great" suggesting conscious willful agency behind the "replacement".
And it is this variation that prompts people to say well then, it is a conspiracy theory.
But actually there are variations on the central theory, that explain the observed facts (from certain perspectives) and the counterfactuals (again strongly driven by personal perspective), by this theory claiming that the explanatory dynamic involved should be characterised "replacement" etc.