The KEY to understanding Jn. 6:37–40 is quite simple indeed and is found in the included context of Jn. 6:39–40.
Jesus declared in Jn. 6:44 that no man could COME unto Him unless he first be drawn by the Father.
Therefore, how do we COME (be drawn) to the Father?
To this end, how do we get FAITH?
Thus, when Jesus declared in Jn. 6:37 that ALL that the Father IS GIVING (present tense) to Him will COME to Him this is indeed true and an absolute ALL. Furthermore, Jesus will NOT banish nor reject a singular one of them in which the Father is giving Him, because Jn. 6:38 reveals Jesus came to do His Father’s WILL and not His own will.
However, Jn. 6:37 has nothing whatsoever to do with irresistible grace but is predicated upon the Holy Spirit drawing sinners to God through the hearing of the WORD of God. Hence, FREE-WILL.
Meanwhile, Jn. 6:39 Jesus declared that of ALL that the Father has given Him HE (I) should LOSE (verb/1st person/aorist/subjunctive/active) none. Hence, Jesus is the "Good Shepherd" (Jn. 10:14-15) and no man can pluck His Sheep out of His hand. (Jn. 10:28-29)
Although, PLEASE take note that Jn. 6:39 does not declare nor insist unconditional grace and that the ALL Jn. 6:37 Sheep given by the Father to Jesus will therefore ALL be unconditionally saved. Thus, Jn. 6:39 simply declares that Jesus Himself (I) will not be the literal causal agent of the LOSS (lose) by any means of ANY of His Father given Sheep.
Therefore, it is the Father's perfect WILL that ALL the Sheep that He "gives" to Jesus, that Jesus should not be responsible for the LOSS of ANY of them, but should raise them up in the last day. However, the Father's perfect will is not always the same WILL of Jesus' FREE-WILL Sheep.
As such, Jn. 6:40 reveals those of the ALL Sheep of Jn. 6:37 & 39 that is actually having eternal life and will be futuristically raised from the dead will ONLY be those who are SEEING (present tense) and BELIEVING (present tense) in Jesus. It is they who are having eternal life (present tense). This is an individual CHOICE to BELIEVE.