There are lots of parking in Kalajoki around anywhere, but best places to start would be at dunes, with sights and services near by. Kalajoki can be too popular in the summer during holiday season or if weather is spectacular.
In the video the beginning is mostly the beautiful coastline with activity possibilityes, golf fields, cottages and towards the end it flows through some documented and undocumented trails in the near by area.
The mountainbiking trails are easy, but can be soft sand occationally so fat or e-bikes can be handy. It is not easy to get lost here with at least 3G coverage and definite area encircled by sea, river and roads. The official markings and crossing paths can be a bit confusing for first timer.