I’ll give you a tip: Wikipedia is your friend. You just have to choose your city and then see who flies there from where. For example: https://en.wikipedia.org/wiki/Detroit_Metropolitan_Airport#Airlines_and_destinations As you can see, no one flies directly to Hawaii. So, you need to check the routing for Honolulu (Assuming that’s your desired destination): https://en.wikipedia.org/wiki/Daniel_K._Inouye_International_Airport#Airlines_and_destinations You can create your routing from DTW through Phoenix on Southwest. You can now book DTW-PHX-HNL on Southwest as a singular ticket or 2 if you need an extra day to make the routing schedule work. (Overnight in PHX with a morning flight to HNL for availability purposes)
If that doesn’t work, you can break up your tickets if you can’t find availability using a single airline. For example, you can book DTW-LAX on United and then LAX-HNL on AA or Hawaiian using BA Avios. Why? Because BA is a Oneworld alliance partner meaning you are using BA Avios but flying on AA/HA metal. BA is also a Chase transfer partner. Here’s an example: https://thepointsguy.com/guide/british-airways-sweet-spot-hawaii/ There are several different cities that you pair HNL with. Most airlines have an interline agreement for your check-in luggage to transfer to your new flight. You won’t get that with Southwest, meaning that you will need to recheck you luggage in LA if you use a different airline on one of your routes. If you are splitting airlines, make sure that you have plenty of time to catch your next flight as you may need to change terminals. You can check beforehand by Googling “What terminal does XXX fly into PHX” to double check yourself. (Or use the # link next to your airline on Wikipedia) Most airport sites will tell you the distance between terminals to plan accordingly.
It just takes some practice. The golden rule is to make sure that all your ducks are in a row, for the class/# of tickets you need, BEFORE transferring any points.