Personally, I'm like, pretty sure that Nikolai loves like a worshipper. To him, John is an ethereal being, someone that is above all laws of beauty. John is the closest thing Nik could devote to. Praises come to him naturally when they are about John, and he could map out John's body in his sleep.
He likes taking care of John, to know that his devotion is happy and well because of him personally. Nik likes grooming John in the bathroom, to have the Brit relax into his arms as he slowly trim his mutton chops (at least that's what I think it's called). He could only watch in silent adoration as John relaxes into the semi normal routine they have, his presence in Nik's kitchen seeming natural. Nik could barely fathom how he had gone so on without John next to him, for such times seem impossible now.
His devotion for John runs deeper than the ocean, and it seems like there is a never ending amount of things Nik could list off about John.
Laswell knows it, the rest of the team knows it, and it seems like only Price is the one oblivious to how much the other adores him. (He interacted with Alejandro for barely less than a day once, and even he knows how devoted Nik is to John). Something about waiting for so long for the man had translated into some form of utter adoration and devotion for him.
John, on the other hand, loves Nik to the point of creation and healing. I dunno how to put this into words, but I'm sure he cut down HEAVILY on drinking and smoking after knowing Nik. He loves the man, so much so that he wants to become a better person for Nik. To grow old and watch your partner grow old next to you seemed like some impossible dream almost a decade ago. And now he sits on the toilet as Nik helps him shave, the russian's movements almost graceful.
John has no doubts that Nik could be with anyone else in the world, with men better than him. But Nik is laying in John's bed, head tucked under John's chin and John finds himself hardly caring. Nik was with him, and that was more than enough.
He also writes little poems for Nik occasionally, words flowing onto the paper freely before he seals it with a kiss. He knows Nik has them hidden somewhere, the russian having said something about the letters being 'too precious for mortal's mind'. He likes tinkering with things, learning extremely useless and niche hobbies so that when winter rolls in, Price could make a sweater for Nik.