Correct, no on-board record, no paper service book (although you can of course buy an aftermarket one and get it signed). As to when the next service is due, the car will tell you what it thinks. 2 problems with that. 1) It may not be on the schedule (long life or annual) that suits your usage. 2) some of the algorithms are crap - I have an 8 month old, 3500 mile car, and it’s decided it needs the oil changed. That’s not going to happen until it’s first birthday in September, and annually thereafter (I dont agree with long-life servicing that can see 2 years between oil changes, but that’s personal).
Of course if it’s a new car on PCP then you just do what it says at a SEAT dealer. If, however, you own your car, as I do, you have a choice. You can use main dealers, or an independent. For decades you have been able to get servicing done anywhere AND keep you warranty safe, as long as the manufacturer schedule is adhered to and genuine parts are used.
I have a local VAG specialist who regularly works on the likes of Audi R8’s, and they pay to access the VAG service system, so can keep an electronic service record up to date. So, if you want that security find a similar indie, or, as you say, use SEAT.