Usando solo due layer linea_a e linea_b e tematizzare usando il GENERATORE DI GEOMETRIE

Unione

UNION( 
make_line(  -- linea a
	start_point(geometry(get_feature_by_id('linea_a',1))), 
	start_point($geometry),
	end_point(geometry(get_feature_by_id('linea_a',1)))),
make_line(  -- linea b
	start_point($geometry), 
	end_point(geometry(get_feature_by_id('linea_a',1))),
	end_point($geometry))
				)

NB: per gli shapefile sostituire 0 al posto di 1; es: ('linea_a',0)

Differenza Smmetrica

SYM_DIFFERENCE( 
make_line(  -- linea a
	start_point(geometry(get_feature_by_id('linea_a',1))), 
	start_point($geometry),
	end_point(geometry(get_feature_by_id('linea_a',1)))),
make_line(  -- linea b
	start_point($geometry), 
	end_point(geometry(get_feature_by_id('linea_a',1))),
	end_point($geometry))
				)

Differnza

DIFFERENCE( 
make_line(  -- linea a
	start_point(geometry(get_feature_by_id('linea_a',1))), 
	start_point($geometry),
	end_point(geometry(get_feature_by_id('linea_a',1)))),
make_line(  -- linea b
	start_point($geometry), 
	end_point(geometry(get_feature_by_id('linea_a',1))),
	end_point($geometry))
				)

DIFFERENCE( 
make_line( -- linea b
	start_point($geometry), 
	end_point(geometry(get_feature_by_id('linea_a',1))),
	end_point($geometry)),
make_line(  -- linea a
	start_point(geometry(get_feature_by_id('linea_a',1))), 
	start_point($geometry),
	end_point(geometry(get_feature_by_id('linea_a',1))))
				)

Intersezione

INTERSECTION( 
make_line( -- linea a
	start_point($geometry), 
	end_point(geometry(get_feature_by_id('linea_a',1))),
	end_point($geometry)),
make_line(  -- linea b
	start_point(geometry(get_feature_by_id('linea_a',1))), 
	start_point($geometry),
	end_point(geometry(get_feature_by_id('linea_a',1))))
				)