see how fast they changed attire when they turned up at NY timeline using that suit

that platform is a quantum realm machine.
u go inside > go anywhere > return

if u return, u will continue the present time in few secs

if u didnt return, u stuck there. with that GPS, u can travel anywhere in the past and turn up at location u set. u cannot go to the future without another 'opened' quantum realm machine.

2014 thanos cannot jump to endgame without portal open.
nebula hv to open it up manually for thanos.

"how old cap return?"

damn. now we talking.. russo plan to do some story telling for future project..

my theory is this..

A: 1950
B: 2023
C: 2066

if quantum open in 2023, it can close in few seconds.
from B, u cannot go C because that is future.
in order from B to go C, u need portal to open in C
but, u can go A because that is ur past.

lets follow steve in endgame.

from B, he jump to A
from A, he didnt return. he stays
if he stays until C, he can jump to B without portal.

the scene where steve jump from NY to 1970 prove this theory.

the rule of quantum realm time travel is = u MUST go into quantum realm

let say u have pym particle with suit and GPS.
u going nowhere.

if other dude open quantum realm machine, u also going nowhere.

you MUST jump into quantum first.MUST.

[NY scene without portal]
1. 2023 jump into portal
2. appear in NY (portal still open)
3. jump to 1970 (portal still open)
4. jump anywhere u want with pym particle as long the portal is open.
5. return to 2023 on platform.

if u are at 3, u can appear anywhere before 2023 because thats where u start.

[Old Cap]
1. jump at 2023
2. appear 1950
3. he didnt return (hulk is 5 sec)
4. since he didnt return, he cannot jump to 2024 onwards
5. if he stays timeline no.2 until after 2024 onwards, he can appear anywhere he like in 2023 because that is his past

no loop.

go watch the NY scene again, this time..

[avengers close up]
imagine this as portal with cap about to jump, 5sec

[cap+tony+scot+hulk appear]
old cap appear in style, ready to sit on the bench doing nothing

there u go ..