There are lots of different routes through Portugal, as you can see from the number of different Camino
Portugues sub-forums.
When people talk about the Central vs Coastal routes, they are talking about routes between Porto and Santiago. There are actually three main routes from Porto to Santiago, with a couple of lengthy variants.
The three main routes, from east to west, are the Central, the Coastal, and the Senda Litoral. The Central more or less goes north from Porto through Barcelos, Ponte de Lima, Valença and crosses the border at Tui. The Coastal also heads north out of Porto but veers west and reaches the coast at Vila do Conde, then heads north sometimes along the coast, sometimes somewhat inland. The Senda Litoral is the one that really follows the seashore, heading out of Porto along the river and then up along the beaches. The three all join together at Redondela in Spain.
The first variant I mentioned is one that passes through Braga, to the east of the normal Central route. I don't believe it gets much traffic and isn't in many of the guidebooks or apps, but it exists. The second variant is the Variante Espiritual (Spiritual Variant) which occurs after the three have joined together. It departs from the main route shortly after Pontevedra and rejoins it at Padron.
I believe the Coastal/Senda routes are a little easier in terms of inclines than the Central, although neither of these is terribly difficult in terms of inclines compared to many other Camino routes.