After these things I looked,
John is doing what he was told to do (Revelation 1:19).
This is the third 'I looked' in the last two chapters. Each time John sees something different.
and the TEMPLE of the TABERNACLE OF TESTIMONY in heaven was opened,
This is symbolically Jesus Christ as the fulfiller of the LAW (TESTIMONY - Exodus 25:16-22). We see the innermost part of the tabernacle containing the ark being made open for this vision. The ark was both the mercy seat and the container of "the LAW". In this verse, there is no mention of the mercy seat (Hebrews 10:1-10). The time for mercy is past; now it is time for "the LAW" through judgment to act.
On earth, the devil tries and succeeds in taking ownership of this part of the temple (Matthew 24:15-28 and Daniel 11:29-35). But this TEMPLE of the TABERNACLE OF TESTIMONY is in heaven where God and the Lamb are in charge.
For continuity, this verse really gets us back into the main thread of Revelation. That is, it is a continuation of the temple opening in Revelation 11:19. Recall that the seven bowls (verse 7) are contained within the seventh trumpet (Revelation 11:15-19).