Using model builder how do i merge polygons, that have their centroid within another polygon?

[SianRennie]

I am new to model builder. I am trying to write a model which selects a catchment (as seen below in dark blue outline) and then merges another feature class, which have their centroid within this catchment area (selected).  It would repeat this process working through all the catchment areas. I have an initial model however this uses select and selection by location tools, as seen below, and does not merge or work through the catchment areas. Any information is greatly appreciated. 

[DanPatterson]

Do you mean merge or dissolve?

Dissolve (Data Management)—ArcGIS Pro | Documentation 

[SianRennie]

Thanks for the prompt. I originally thought merge would provide the results i require however after looking at the dissolve tool this would work. As there was not a common attribute in the selected feature class i could use to dissolve polygons i have carried out a spatial join to assign the catchment attributes to a point/centroid layer and then a join on the selected features polygon layer.This has given me an attribute that i can dissolve the polygons by. I have done this step by step, not within model builder, however it has allowed for the entire feature class to be dissolved, by the appropriate attribute, as opposed to a single catchment area at a time. I appreciate this may not be the optimal process and i would like to be able to use model builder so that i can change the data and not rerun the entire process manually. 

[DanPatterson]

That is what modelbuilder is for.... chain your steps together and set model parameters as you go.