Is my social security distributions part of AGI?

Yes, the taxable portion of Soc Sec is part of your AGI.  See Form 1040, lines 6b and 11.

MAGI has different definitions for different purposes.  Why specifically are you asking?

2020 Form 1040 (irs.gov)

Depends on your income level.  The higher your income, the more of your Social Security benefits are taxable.  You can check line 6b of your 1040 to determine if any of your Social Security is included in income.  You can review your 1040 in the following steps:

1. While in your TurboTax account, select Tax Tools in the left margin
2. Select Tools
3. Select View a Tax Summary
4. In the left margin, select Preview my 1040
5. Scroll down and find line 6B
6. Select Back when completed

MAGI is modified adjusted gross income.  Generally, it is your adjusted gross income from line 11 of the 1040 plus non-taxable income like like tax exempt interest from line 2a