So to make sense of the last 5 minutes in the order as they appear: the 'older' Olivia tells younger Olivia that she has a purpose -- what Cassie in 1971 told her that she can have a purpose in life. It's a causality loop
Those three Army of the 12 Monkeys followers sent by Olivia to beat up Cassie in her past. Causality loop.
Olivia's advisor whom we see in this episode was sent to 1961 to guide Albert Kirschener to create the Messengers using Mantis' (her name) eggs, a former Messenger herself (in 2x10 Fatherland). This is how Kirschener creates Olivia. This is why she displays uncharacteristic strength and immunity to the Kalavirus, she was made to be immune from disease as the Messengers themselves. Fast forward to 2015 (recall 1x13 Arms of Mine), we see those very same Messengers as babies and then as adults... they were created from Olivia's DNA and Olivia being from Mantis (when she was stuck in 1944 to live her life until 1971). Yup, causality loop.
"Find Cassandra Reilly, prepare her". This was in 1x06 where Cassie was kidnapped (from the Night Room) and where we see her being drugged by the red tea by Olivia to take her on a trip to the Red Forest. It was the present Olivia herself who gave Season 1 Olivia orders to make Cassie drink the red tea.
It was also Olivia herself who possessed Cassie and took control of her body to hold Sam, Ramse's son at hostage and the one to sabotage the Machine in 2x7 (which caused Dr. Eckland's death in fixing it).
We see then Cole being confronted with the Witness when his consciousness was travelling in the timestream after drinking the red tea to remove the 1958-59 time he spent with Cassie in the house of Cedar & Pine in 2x13 - Memory of Tomorrow in order to stop the 1957 paradox.
All of this has showed how things came to be, and as the running theme of the show, our heroes are stuck in a 'great Djinn'... endless loops that are unbreakable. Nothing has changed, only how we know now and then with how everything that has happened since the start of the show.
Also, we see how Mantis dies in 1971, that we first see in 2x4 Emergence... dying from injuries from her gunshot wound having shot by Cassie in this episode. (We assumed before then she probably died from old age but nope lol)
"I will ensure my own creation, and the creation of others, for the great Djinn". This is what this episode was about, showing the origins of the Witness and how every events are pre-ordained.