TypeError when adding QgsGeometry object to canvas using PyQGIS

Question

How do we add a QgsGeometry in memory object using QgsProject.addMapLayer() or other PyQGIS code?

ERROR

TypeError: QgsProject.addMapLayer(): argument 1 has unexpected type 'QgsGeometry' buffer <QgsGeometry: Polygon ((336097.91999999998370185 8173802.9299999997019767

CODE Snippet

line_layer = QgsProject().instance().mapLayersByName('b5_Dissolved_Roads_clip')[0]
chainage_layer = QgsProject().instance().mapLayersByName('a3_Roads_10m_Chainage')[0] #Change name to match your chainage layer

fids = []
feats = [f for f in line_layer.getFeatures()]
start_end=-1 #0 for start, -1 for end
field_to_update='End_Chainage'
print(datetime.datetime.now())
print (feats)

for feat in feats:
    name = feat['Location']
    print (name)
    c_sel = [p for p in chainage_layer.getFeatures() if p['Location'] == name]
    print (c_sel)
    geom = feat.geometry().asPolyline()
    end_point = QgsGeometry.fromPointXY(geom[start_end])
    buffer = end_point.buffer(20, 4) #Xm buffer with 4 segments
    QgsProject.instance().addMapLayer(buffer)

Running QGIS 3.26.3-Buenos Aires

Answer 1

Just to avoid keeping the question without an answer, as commented by @GeorgeC, most of the answer of solution 2 in Display buffer in a new layer - PyQGIS? solve the problem. Specifically, adding the following code will load the buffer layer correctly:

buffer_lyr=QgsVectorLayer("Polygon?crs=epsg:28355","buffer", "memory")     
f = QgsFeature()     
f.setGeometry(buffer)     
buffer_lyr.dataProvider().addFeatures([f])     
QgsProject.instance().addMapLayer(buffer_lyr)

The full code will be like this:

line_layer = QgsProject().instance().mapLayersByName('b5_Dissolved_Roads_clip')[0]
chainage_layer = QgsProject().instance().mapLayersByName('a3_Roads_10m_Chainage')[0] #Change name to match your chainage layer

fids = []
feats = [f for f in line_layer.getFeatures()]
start_end=-1 #0 for start, -1 for end
field_to_update='End_Chainage'
print(datetime.datetime.now())
print (feats)

for feat in feats:
    name = feat['Location']
    print (name)
    c_sel = [p for p in chainage_layer.getFeatures() if p['Location'] == name]
    print (c_sel)
    geom = feat.geometry().asPolyline()
    end_point = QgsGeometry.fromPointXY(geom[start_end])
    buffer = end_point.buffer(20, 4) #Xm buffer with 4 segments
    buffer_lyr=QgsVectorLayer("Polygon?crs=epsg:28355","buffer", "memory")     
    f = QgsFeature()     
    f.setGeometry(buffer)     
    buffer_lyr.dataProvider().addFeatures([f])     
    QgsProject.instance().addMapLayer(buffer_lyr)