For such a generic question the answer is really "it depends"! #1, would somewhat depend upon code version being used. Original Juniper EX default VLAN was VLAN 0, namely the bit-bucket. This was later change to VLAN 1 with vlan-name being 'default'. So if you are on newer code, likely 12.3R9+, default on switch 2, would be VLAN 1, name = default. Note that some Junos VLAN config uses name, not vlan-id/number.
For AE on switch 2, this would be non-tagged. The tagged packets (with VLAN ID 20) received from switch 1, should therefore become part of native-vlan-id VLAN.
For traffic from switch 1 to switch 2, this would be untagged packets, going to a tagged port. Junos by default, drops these type of packets (untagged received on tagged interface) but this can be over-written by config. These packets would also be associated with native-vlan-id VLAN.
As well, from your description, unless VLAN 20 on Switch 1 has other interface members besides the tagged AE, then nothing is probably going to be transmitted go out that AE interface, outside of [maybe?] untagged CPU/RE generated multicast/broadcast traffic, which depend upon what is under Protocol Stanza. Something like LLDP or RSTP for example.
All of this basic VLAN operation which almost all switching vendors in 2018 perform similar functionality, just that config (and default setings) may be different.