Bukhari

Hadith Commentary:
Abdullah ibn Umar (radi Allahu anhuma), due to weakness in his later years, would sit cross-legged (with both legs folded) during the tashahhud. This was solely due to his excuse (i.e., physical inability); otherwise, the Sunnah method is that the right foot remains upright and the left is spread out, and one sits upon it—this is called tawarruk. This is also the Sunnah for women. The correspondence between the chapter and the hadith is evident.

Hadith Commentary:
(1)
In this narration, it is not clarified whether, after spreading the left foot, he would sit upon it or perform tawarruk; however, in the narration reported by Imam Malik, it is clarified that after spreading the left foot, he would not sit upon it, but rather would sit on his hip.
From this detailed narration, it is understood that the method described in this narration pertains to the second tashahhud, as is explicitly stated in the narration of the Muwatta itself, that in the final tashahhud, he (sallallahu alayhi wa sallam) would do so.
Even if the narration presented in Sahih al-Bukhari is interpreted as referring to the first tashahhud, there is no problem, as Imam al-Nasa’i rahimahullah has reported from Abdullah ibn Umar radi Allahu anhu that the Sunnah method of sitting in prayer is to sit on the right foot.
(Sunan al-Nasa’i, al-Tatbiq, Hadith: 1158)
(2)
It should be noted that sitting cross-legged (charzanū) in prayer is not correct, as Abdullah ibn Mas‘ud radi Allahu anhu states that he would prefer sitting on coals to sitting cross-legged in prayer. However, due to an excuse, one may sit cross-legged in either voluntary (nafl) or obligatory (fard) prayer.
(Fath al-Bari: 2/396)
And Allah knows best.