The $880 billion is the amount that the House Republican budget plan adopted on February 25 directs the House Energy and Commerce Committee to find ways to cut. That committee has jurisdiction over Medicare, Medicaid, and a few smaller programs.
Republicans have ruled out Medicare cuts. With Medicare off the table Medicare accounts for 93% of the funding that the House Energy and Commerce Committee can cut to reach their $880 billion target. That other 7% is way way below $880 billion meaning that to actually cut $880 billion from programs that committee has jurisdiction over must involve cuts to Medicaid.
As far as SS goes, the current Republican party platform does say it is safe: "FIGHT FOR AND PROTECT SOCIAL SECURITY AND MEDICARE WITH NO CUTS, INCLUDING NO CHANGES TO THE RETIREMENT AGE" (yelling from the original). (BTW, note that they mention Medicare but not Medicaid).
However there is a good reason to not feel reassured by that. The Republican Study Committee, the largest conservative caucus in the House whose membership is around 60+% of House Republicans had repeatedly said that there are three ways to address ~2034 expected insolvency of the SS Trust Fund which are (1) increase revenue to the fund, (2) cover shortfalls from the general budget, and (3) reduce benefits. They also repeatedly say that #1 and #2 are off the table.
If #1 and #2 are off the table, and Congress does not reduce benefits, then there will be a benefit cut when the trust fund runs out. I suppose technically they could argue that letting that happen through inaction rather than passing legislation to cut benefits would mean that they didn't violate their promise not to cut benefits.