Train clarification please and tourist card guidance

Oriente is a thru station with trains coming from elsewhere.
The station has 4 levels , the top level is the train platforms, station level 3 has the ticket booths where you can ask which platform for the train you want ( I believe it's line 5 but other trains also use the same line).
You'll have only a few minutes to board the train.
Level 2 is street level.
Level 1 ( lowest level) is the Metro Red line from the airport.

Lisboa Santa Apolonia station is the starting station for most of the AP trains to Porto ( the exception is 2 AP trains that start in Faro ).
If you prefer to board the train at the starting station you'll have extra time to board but you'll have to make your way to Santa Apolonia either by taxi or Metro and that requires 2 different Metro lines.

Metro lines map.
https://www.metrolisboa.pt/viajar/mapas-e-diagramas/

You can purchase the tickets in person at the station or online - I usually allocate between 2 to 3 hours after scheduled arrival.
https://www.cp.pt/passageiros/en

Viva Viagem card has been replaced by Navegante Ocasional card, you can load tickets or money into the card to use the zapping.
https://www.metrolisboa.pt/en/buy/viva-viagem-card/

Lisboa card has transportation and free or discounted entrance to some museums/attractions, you'll have to determine if it's worth it to you.https://lisboa-card.com/included-attractions-in-lisboa-card/

Porto has 2 Andante cards , the regular Andante card uses a zone system or the Andante Tour card that is valid in all zones.
https://www.introducingporto.com/andante-card

We usually use the HoHo bus or the Andante Tour card to avoid issues.

Wrote a trip report that has some photos of the AP train ( towards the beginning of the report) and Oriente station ( towards the end of the report) if you want to check it out.
https://www.fodors.com/community/europe/extended-trip-to-portugal-1711933/

Last time I took the Lisbon Metro it was two lines from the airport (that is one change) to Santa Apolonia. The red line to it's end station then the blue line southbound to it's terminus.
Or get off at Oriente and get any local CP train from Oriente main line south into Santa Apolonia- 3 or 4 an hour- no reserved seats, board at any door.
Then board the Porto train at it's origin of Santa Apolonia, with time to look around the station. The worst thing about Oriente is that it is brutally modern, the antithesis to a typical Portugese station. The best thing is that it is a modern multi-modal station.

As for any train on arrival day almost anywhere I suggest buying your ticket on arrival, to avoid stress.

You may not use public transport that much. I rarely use Uber or Bolt anywhere, but in Lisbon (and Porto) it was our main mode of transport. Cheap, direct, fast. The times that the limited metro lines worked out, we just tapped in and out with our credit cards.

Thank you so much! These are very helpful responses, especially the trip report. I have a better idea of what to expect.