For uplink:   both QSFP+ ( 40G  ) or SFP+ ( 10G ) can be used

SPF, SPF+, QSPF, QSFP+, SFP28, QSFP28 and DAC

https://www.juniper.net/documentation/en_US/release-independent/junos/topics/topic-map/ex3400-system-overview.html#jd0e102

Virtual Chassis

You can interconnect a maximum of 10 EX3400 switches to form a Virtual Chassis. You can operate these interconnected switches as a single, logical device with a single IP address.

• QSFP+ ports configured as VCPs by using QSFP+ transceivers

• NOTE

  You cannot form a Virtual Chassis by using SFP transceivers.

  Uplink ports on the front panel configured as VCPs by using SFP+ transceivers

By default, the QSFP+ ports are configured as VCPs.