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a_name = "A" # Layer name in QGIS
b_name = "B"
l_name = "L"

fld_id = "id" # Id field to join B and A

# Load layer
# QGIS layer name must not be duplicated
layer_a = QgsProject.instance().mapLayersByName(a_name)[0]
layer_b = QgsProject.instance().mapLayersByName(b_name)[0]
layer_l = QgsProject.instance().mapLayersByName(l_name)[0]

# Build spatial index in case a large number of line is available
index = QgsSpatialIndex(layer_l.getFeatures())

with edit(layer_l):
    for feature in layer_a.getFeatures():
        geom = feature.geometry()
        
        # Get candidates based on bounding box.
        candidate_ids = index.intersects(geom.boundingBox())
        request = QgsFeatureRequest().setFilterFids(candidate_ids)
        
        # Fetch candidates
        lines = [f for f in layer_l.getFeatures(request)]
        
        # Assuming a point from A intersect only one line
        if len(lines) > 1:
            raise Exception("Too many candidates")
        elif len(lines) == 0: # Skip this geom
            continue
        else: # For one match
            # Find the vertex to move
            line = lines[0]
            vertex, vtx_idx, _, _, _ = line.geometry().closestVertex(geom.asPoint())
            # Find the corresponding B point
            exp = f"{fld_id} = {feature[fld_id]}"
            request = QgsFeatureRequest().setFilterExpression(exp)
            # Assuming one match
            feature = [f for f in layer_b.getFeatures(request)][0]
            point_b = feature.geometry().asPoint()
            new_line = line.geometry().asPolyline()
            new_line[vtx_idx] = point_b
            
            # Update geometry
            new_geom = QgsGeometry().fromPolyline([QgsPoint(v) for v in new_line])
            line.setGeometry(new_geom)
            layer_l.updateFeature(line)