shine, illuminate, cover, envelop
燾 centers on light that spreads over something: shining, illuminating, and by extension covering or enveloping like a blanket of light.
燾 combines 壽 (long life, here likely phonetic) with 灬 (fire), suggesting light or fire that endures and spreads. The exact historical development is uncertain.
The top 壽 suggests something long-lasting, and the bottom 灬 is fire. Picture a long-burning fire that shines and covers everything with its warm light.
For トウ, imagine a tall tower (トウ) with a fire on top that shines and covers the land with light: tower -> トウ.