The younger squad don't count towards the 25 man registration limit for the first team. The regulation for Covid says minimum 13 + 1GK from the first team squad plus any U23s who have previously been named in a match day squad during this season or the last, ( termed 'suitably experienced' ). If NUFC have not been listing U23s in the 18 man squads then they don't count. Saints have been doing so, for instance the starting 11 and subs for the 9-0 at OT, so we wil find it harder to exploit the Covid rule.
In the Championship, and below, the rule is 13+1 from the combined first team and reserves, regardless of experience.