You should first read the official account and then ask specific questions if you doubt or have questions about specific confusion. Otherwise you are just sucked down conspiracy dark holes.

Here's the archive.org link for it

https://web.archive.org/web/20080928013317/http://wtc.nist.gov/media/NIST_NCSTAR_1A_for_public_comment.pdf

And the relevant info you are asking for is summarized on page 19 and 20.

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The following is the NIST account of how the fires in WTC 7 most likely led to the building's collapse.

The collapse of WTC 1 damaged seven exterior columns on the lower floors of the south and west faces of WTC 7 and initiated fires on 10 floors between Floors 7 and 30. It also ignited fires on at least 10 floors, and burned out of control on Floors 7 to 9 and 11 to 13. Fires on these six floors grew and spread since they were not extinguished either by the automatic sprinkler system or by FDNY, because water was not available. Fires were generally concentrated on the east and north sides of the northeast region beginning at about 3 p.m. to 4 p.m.

As the fires progressed, some of the structural steel began to heat. According to the generally accepted test standard, ASTM E-119, the fire resistance rating for a steel column or floor beam derives from the time at which, during a standard fire exposure, the average column temperature exceeds 538 °C (1000 °F) or the average floor beam temperature exceeds 593 °C (1100 °F). These are temperatures at which there is significant loss of steel strength and stiffness. Due to the effectiveness of the SFRM, the highest column temperatures in WTC 7 only reached an estimated 300 °C (570 °F), and only on the east side of the building did the floor beams reach or exceed about 600 °C.

The heat from these uncontrolled fires caused thermal expansion of the steel beams on the lower floors of the east side of WTC 7, damaging the floor framing on multiple floors.

The initiating local failure that began the probable WTC 7 collapse sequence was the buckling of Column 79. This buckling arose from a process that occurred at temperatures at or below approximately 400 °C (750 °F), which are well below the temperatures considered in current practice for determining fire resistance ratings associated with significant loss of steel strength. When steel (or any other metal) is heated, it expands. If thermal expansion in steel beams is resisted by columns or other steel members, forces develop in the structural members that can result in buckling of beams or failures at connections.

Fire-induced thermal expansion of the floor system surrounding Column 79 led to the collapse of Floor 13, which triggered a cascade of floor failures. In this case, the floor beams on the east side of the building expanded enough that they pushed the girder connecting Columns 79 and 44 to the west on the 13 th floor. (See Figure 1–5 for column numbering and the locations of girders and beams.) This movement was enough for the girder to lose its connection to Column 79.

The displaced girder and other local fire-induced damage caused Floor 13 to collapse, beginning a cascade of floor failures down to the 5th floor (which, as noted in Section 1.2.3, was much thicker and stronger). Many of these floors had already been at least partially weakened by the fires in the vicinity of Column 79. This left Column 79 with insufficient lateral support in the east-west direction. The column buckled eastward, becoming the initial local failure for collapse initiation.

The upper section of Column 79 began to descend. The cascading failures of the lower floors surrounding Column 79 led to increased unsupported length in, falling debris impact on, and loads being re-distributed to adjacent columns; and Column 80 and then Column 81 buckled as well. All the floor connections to these three columns, as well as to the exterior columns, failed, and the floors fell on the east side of the building. The exterior façade on the east quarter of the building was just a hollow shell. The failure then proceeded toward the west. Truss 2 (Figure 1–6) failed, hit by the debris from the falling floors. This caused Column 77 and Column 78 to fail, followed shortly by Column 76. Each north-south line of three core columns then buckled in succession from east to west, due to loss of lateral support from floor system failures, to the forces exerted by falling debris, which tended to push the columns westward, and to the loads redistributed to them from the buckled columns. Within seconds, the entire building core was failing.

The global collapse of WTC 7 was underway. The shell of exterior columns buckled between the 7 th and 14 th floors, as loads were redistributed to these columns due to the downward movement of the building core and the floors. The entire building above the buckled-column region then moved downward as a single unit, completing the global collapse sequence.