ArcMap allows you to publish layers to the web. You can then add these to a new Web Map and configure the web map in ArcGIS Online. You may need to update labels and symbology etc.
https://desktop.arcgis.com/en/arcmap/latest/map/publish-map-services/share-gis-resources-through-pub...
ArcGIS Pro is better at sharing to ArcGIS Online. It has access to Arcade for labels etc, and better supported symbology for online compatibility. You can share layers like in ArcMap,
https://pro.arcgis.com/en/pro-app/latest/help/sharing/overview/introduction-to-sharing-web-layers.ht...
but you can also create the Web Map as part of publishing (with things like basemap).
https://pro.arcgis.com/en/pro-app/latest/help/sharing/overview/share-a-web-map.htm
If this is a personal project, I would recommend using ArcGIS Pro. Everyone should be using ArcGIS Pro unless there is a specific need to stay on ArcMap (company has yet to migrate or you have dependency).
Once you have published layers and have added them to a web map, I would recommend creating a Web Mapping Application. This allows you to add branding, purpose and functionality to your map (compared to the Web Map Viewer, which is more of a building tool). It's like adding a layout to a map in ArcMap or ArcGIS Pro. Which application you use depends on functionality.
- ArcGIS Instant apps = quick, easy and purposeful
- ArcGIS Story Maps = tell a story and add media
- ArcGIS Dashboards = data and infographics
- ArcGIS Experience builder = a more powerful viewing experience
You could use configurable map templates or the ArcGIS Web App Builder but again I would recommend of the above as they are the newer applications that leverage the 4.x API. The others use 3.x API and are being phased out, so you may as well use the newer ones for functionality and longevity.
