An access port is a Layer 2 switch interface mode intended for an endpoint that belongs to a single VLAN. Traffic on an access port is associated with exactly one VLAN, and frames are typically transmitted and received untagged on the wire. The switch internally maps that untagged traffic into the configured access VLAN, placing the endpoint into the correct broadcast domain. This behavior is widely used for server access, management ports, out of band devices, and any endpoint that does not tag VLANs.
By contrast, trunk ports are designed to carry multiple VLANs simultaneously, usually with 802.1Q tagging, and are typically used between switches, to routers, to virtualization hosts, or to appliances that handle multiple VLANs. That is why assigning multiple VLANs to an access port is not the standard access mode behavior.
An access port does not have to connect to a router or a firewall. It can connect directly to any Ethernet endpoint. Routing between VLANs is provided by a Layer 3 interface such as an IRB interface on the switch or an external routed device, but that is independent of whether the endpoint connects on an access port.
Verification sources from Juniper documentation
https://www.juniper.net/documentation/us/en/software/junos/multicast-l2/topics/topic-map/bridging-and-vlans.html
https://www.juniper.net/documentation/us/en/software/junos/multicast-l2/topics/task/interfaces-configuring-ethernet-switching-access.html
