That’s correct - farmOS uses OpenLayers to build its maps.
(And worth mentioning: we are in the process of building a standalone farmOS-map.js library, which will be used in both farmOS itself as well as in the Field Kit mobile app. More info here: Starting work on farmOS-map library)
Can I replace the default view (Google Maps) with my own OpenLayer map?
There is a lot of flexibility for changing the way the maps in farmOS work, although right now it requires some knowledge of both OpenLayers and the Drupal OpenLayers module. Once we migrate to the new farmOS-map.js library, it will be a lot simpler - that’s the hope anyway. 
Can I upload my own Openlayer map to the farmOS via the web interface? OpenLayer map I create using qgis2web plugin through QGIS.
No. farmOS does not host its own map layers. But if you have a custom layer that you can host somewhere else (eg: on MapKnitter.org, or by hosting your own GeoServer or MapServer) then you can include it in the farmOS map.
Currently the farmOS map includes OSM (OpenStreetMap) and Google Hybrid (satellite+roads) as base layers. It also includes an NRCS module which adds the NRCS Soil Survey layer to maps, and a MapKnitter module which allows you to include a custom aerial imagery layer that is hosted on MapKnitter.org. So it relies on those services for tile hosting.
But if you’re familiar with hosting tiles, then it wouldn’t be too hard to include your hosted layers in farmOS - especially once the new farmOS-map.js library is integrated!