I’m not sure what ground transportation tickets to buy for my three week trip to Netherlands and Belgium. My route is Amsterdam to Antwerp to Moddergat NE to Amsterdam. In between I will be traveling within cities and exploring surrounding areas. Should I buy Eurostar for to/from countries and separate within country travel tickets or is there a combined pass?
You can use the Dutch rail site to buy tickets :
https://www.ns.nl/en
The Eurostar is a little faster but the tickets are only good for a specific train and time and come with a reserved seat and the prices go up as capacity shrinks. The Dutch trains are mass transit, meaning any ticket you buy for a specific route is good for any train at any time that day and prices are fixed.
Moddergat is on the northern Frisian coast. A quick look at the public transport travel planner website https://9292.nl/en shows that it does not have a train station with the closest being in Leeuwarden (and from there the bus). The Eurostar website only seems to allow you to procure tickets to Schiphol or Amsterdam from where you can change trains in the Leeuwarden direction. https://www.nsinternational.com/en though allows you to purchase tickets from Antwerp to Leeuwarden.
On the bus you can tap in and tap out with your credit card (not Amex): https://www.ovpay.nl/en. This would be the easiest way to then further explore the Netherlands using public transport. It looks Arriva is servicing that bit of Friesland and they participate in OVpay. This would also be the easiest option (tap in and tap out with your credit card) when going from Moddergat to Amsterdam.