I notice some adjectives do not agree with their nouns, eg. 'Un jugador estrella'. Am I right that this is because 'estrella' is a word in itself and cannot be changed? I also notice this with the Covid 19 so-called bubble groups or classroom pods. It is 'unos agrupamientos burbuja'. This is singular instead of being plural. Why?
Also, I noticed in a recent edition of El País, the following:
La voluntad de mantener abiertas las actividades educativa y economica.
Again, this is about the coronavirus and does not agree. Why? I am unable to copy and paste the full context.