What is the correct way of defining 1:n relations between two models model_a and model_b of the following form?

model_a { id: 1 }

model_b { id: 2 }

model_a 1 : n model_b

case 1: only model_a contains a reference to model_b

model_a { id: 1, model_b_ids: [2] }
model_b { id: 2 }

case 2: only model_b contains a reference to model_a

model_a { id: 1 }
model_b { id: 2, model_a_id: 1 }

case 3: both models referencing each other

model_a { id: 1, model_b_ids: [2] }
model_b { id: 2, model_b_id: 1 }

1. Which relation (hasOne, belongsTo, hasMany, belongsToMany, etc) I have to use for each model in each case?

2. Which function (associate, save, attach) if have to use to connect two instances?