It's not the construction, but the verb "pasar" which must be mastered.
"Pasársela (haciendo algo)" ("pasárse
lo" in Spain and probably other regions that aren't Mexico) means to spend one's time doing something.*
· Se la pasa dándome órdenes.
-> She keeps giving orders to me all day long.
· Durante el confinamiento me la pasé viendo televisión.
-> During lockdown I spent all the time watching TV.
· Cuando íbamos a casa del abuelo, nos la pasábamos jugando en el jardín.
-> When we visited grandpa, we did nothing but play in the garden.
"Pasar(se)", in the other example, means that something is more than sufficiently advanced or ready.
· Se te pasa el arroz. -> This is an idiom meaning that a woman is getting too old to get married and/or have children.
-> Literally, it means that the rice is getting overcooked.
· Se pasó el jugo. No te lo tomes.
-> The juice expired. Don't drink it.
· Esa sopa está pasada. Te vas a enfermar si te la comes.
-> That soup has gone bad. You'll get food poisoning if you eat it.
· Estos plátanos todavía no sirven para el pan. Necesito que se pasen un poco.
-> These bananas can't be used for the bread yet. I need them to be overripe.
"Se te nota" is a completely different construction and has nothing to do with the previous examples, as this is an impersonal sentence: there is no subject who sees the other person is nervous.
Take a look at
this conversation. You may find it useful to see different uses of "se".
*The "la" or "lo" work as a direct object pronoun, although what they replace is unspecified. Many verbs take this kind of construction.