Hi Alex,
When you finish a topic but at that time you come up another thing needs to say, then you may use “ling wai”.
e.g. wo ming tian qu Beijing. Ling wai, wo de meimei ye hui yi qi qu= I will go to Beijing tomorrow. By the way, my sister will go with me.

We usually use qi ta as an adjective.
e.g. I have other friends except you= wo you qi ta de peng you.

qi ta de ren/ qi ta de wen ti= other persons/ other questions

“bie de” has the same use as “qi ta”. For example, bie de hua4 ti2= other topics; qi ta de fang1 fa3= other ways