Are you in Pro or one of the web-based platforms?
Pro maps have a coordinate system you can set yourself, it defaults to whatever your first layer or basemap uses. You'll have to keep track of what datum transformations are in play but otherwise it'll try and sort itself out.
Web maps and anything derived from them (EXB, Field Maps etc.) pin the coordinate system to the basemap. This means if you don't want datum transformations, you'll have to ensure the basemap was published with the same GCS (and PCS ideally for performance reasons) as your layers. The standard ESRI basemaps are Web Mercator only so you'll have to source, process, host and serve your own basemaps if you want to go down this route.
Alternatively, you'll need to store your layers as Web Mercator before you publish them out, then post-process the data to a projected and transformed copy before you run proper analysis and cartography.
