• To be enrolled in the safety run-in, patients must have an advanced soft tissue sarcoma (not otherwise specified \[NOS\]). To be enrolled in the ASPS cohort, patients must have histologically or cytologically confirmed alveolar soft part sarcoma that is not curable by surgery

⁃ Patients must have measurable disease, defined as at least one lesion that can be accurately measured in at least one dimension (longest diameter to be recorded for non-nodal lesions and short axis for nodal lesions) as ≥ 20 mm (≥ 2 cm) by chest x-ray or as ≥ 10 mm (≥ 1 cm) with CT scan, MRI, or calipers by clinical exam.

• Patients with unresectable, metastatic and measurable ASPS that is resistant/refractory to ICI treatment will be eligible for the ASPS cohort of this study if they show clinical and/or radiological evidence of disease progression after receiving prior ICI therapy (including history and increasing physical symptoms). On-study documentation will include a physician's rationale that supports evidence of clinical disease progression (i.e., increasing tumor pain)

• Age \>= 12 years. Because no dosing or adverse event data are currently available on the use of atezolizumab in combination with selinexor in patients ˂ 18 years of age, children \< 12 years of age are excluded from this study

• Eastern Cooperative Oncology Group (ECOG) performance status =\< 2 (Karnofsky \>= 70%)

• Absolute neutrophil count \>= 1,000/mcL

• Platelets \>= 100,000/mcL

• Hemoglobin \>= 9 g/dL

• Total bilirubin =\< 1.5 x institutional upper limit of normal (ULN) (however, patients with known Gilbert disease who have serum bilirubin level =\< 3 x ULN may be enrolled)

• Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase \[SGOT\])/alanine aminotransferase (ALT) (serum glutamic pyruvic transaminase \[SGPT\]) =\< 2.5 x institutional ULN or =\< 5 x ULN for patients with liver metastases

• Serum creatinine =\< 1.5 x institutional ULN OR creatinine clearance \>= 30 mL/min/1.73 m\^2 by Cockcroft-Gault

• Serum albumin \>= 2.5 g/dL

• Baseline sodium (Na+) \>= 130 mEq/L

• Human immunodeficiency virus (HIV)-infected patients on effective anti-retroviral therapy with undetectable viral load within 28 days are eligible for this trial

• Administration of selinexor or atezolizumab may have an adverse effect on pregnancy and poses a risk to the human fetus, including embryo-lethality. Female patients of child-bearing potential and male patients must agree to use highly effective contraception (hormonal or barrier method of birth control; abstinence) prior to study entry, for the duration of study participation, for 90 days after the last dose of selinexor, and for 150 days after the last dose of atezolizumab, whichever is longer. Breastfeeding is not allowed while on selinexor or for 90 days after the last dose of selinexor. Should a woman become pregnant or suspect she is pregnant while she or her partner is participating in this study, she should inform her treating physician immediately

• Ability to understand and the willingness to sign a written informed consent document

• Willingness to provide biopsy samples for research purposes, except patients enrolled on the safety run-in. Patients that cannot be safely biopsied may be considered for the study upon discussion with principal investigator

• Ability and willingness to swallow pills

• Vaccines intended to prevent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and coronavirus disease 2019 (COVID-19) are allowed