Your question revolves around understanding two ideas.
The first is what it means in Bereshit 25:5 that Avraham gave to Yitzchok all that he possessed.
The second is what these other gifts mentioned in Bereshit 25:6, which he gave to his children from the handmaidens are.
In this particular context, All is referring to the usage as it is brought in the written Torah to Bereshit 9:3
כָּל־רֶ֙מֶשׂ֙ אֲשֶׁ֣ר הוּא־חַ֔י לָכֶ֥ם יִהְיֶ֖ה לְאָכְלָ֑ה כְּיֶ֣רֶק עֵ֔שֶׂב נָתַ֥תִּי לָכֶ֖ם אֶת־כֹּֽל׃
Following Noach's offering to G-d after the flood, G-d blessed Noach and gave to him All (כל). It's meaning is life which is associated in the posuk with green grass which grows through photosynthesis. This is the path of life, meaning the path of Torah and light, like Mishlei 6:23, נר מצוה ותורה אור. This same possession is what Avraham bequeathed to his son from Sarah, meaning Yitzchok. Avraham inherited this possession through his ancestor Shem, the son of Noach.
Concerning the other gifts, Rabbi Yirmiyahu Bar Abba as quoted in Sanhedrin 91a calls them Shem Tumah. Rashi to this expression says it means working with demons.
This all sounds very esoteric. But it is really referring to a general principle found within Torah teaching, for example in Sefer Ma'arechet Elokut, that there are two categories of influence within creation. Those of life (חי) and those of destruction and decay (הרס).
For those descendants of Avraham and Sarah, their path and focus is to be concerned with life and what leads to its increase and diversification.
The others were given the gifts related to destruction and decay. When understanding these gifts, it is easy to only see the more extreme levels of this which are truly frightening. Those which are referenced, for example in Shemot 7:11
וַיִּקְרָא֙ גַּם־פַּרְעֹ֔ה לַֽחֲכָמִ֖ים וְלַֽמְכַשְּׁפִ֑ים וַיַּֽעֲשׂ֨וּ גַם־הֵ֜ם חַרְטֻמֵּ֥י מִצְרַ֛יִם בְּלַהֲטֵיהֶ֖ם כֵּֽן׃
and in Shemot 22:17
מְכַשֵּׁפָ֖ה לֹ֥א תְחַיֶּֽה׃
We even mention them in the Hagaddah of Passover when discussing the 3,280 Angels of Destruction (מלאכי חבלה,מלאכי המשחית) that came to wipe out Mitzrayim at the time of the Exodus. They are alluded to in the written Torah by the word fist (אגרף) like in Shemot 21:18. The Hebrew word stands for 3 times 1000 plus 280 for a total of 3280 Angels of Destruction.
וְכִֽי־יְרִיבֻ֣ן אֲנָשִׁ֔ים וְהִכָּה־אִישׁ֙ אֶת־רֵעֵ֔הוּ בְּאֶ֖בֶן א֣וֹ בְאֶגְרֹ֑ף וְלֹ֥א יָמ֖וּת וְנָפַ֥ל לְמִשְׁכָּֽב׃
But think of the ideas of decomposition, rotting, fermentation and molds (see also Destroying Angels and Death Caps). They are all levels of destruction, meaning they are moving away from life, not toward it.
And even to this day, looking at the special wisdoms of the far East (where Avraham sent these descendants) like in Japan and Korea, this wisdom is found for example with Kimchi (salt smoked vegetables), Tamari and Shoyu (fermented soy beans and wheat) and Katsuobushi (smoked, molded fish) which is used in Dashi.
So what Avraham was giving to his children from the handmaidens was a means to make a living in the world like all good parents. He knew they were not to have a claim in the inheritance that belonged to Yitzchok (What would ultimately be forbidden to Yitzchok's descendants.) and so gave them what remained. He blessed them.