Combining bootstrap and angular materials might pose some challenges, but there are separate issues in your layout that need attention unrelated to this mixture.
Firstly, ensure the mat-label tag is enclosed within the mat-form-field element
<mat-form-field class="example-full-width" appearance="outline">
<mat-label for="country_ref_id">Country Name<em style="color: red;">*</em></mat-label>
<mat-select class="form-select" data-style="btn-primary" formControlName="country_ref_id" name="country_ref_id" id="country_ref_id">
<mat-option value="">Select Country</mat-option>
<mat-option [value]="Country.id" *ngFor="let Country of CountryList">{{Country.country}}</mat-option>
</mat-select>
</mat-form-field>
Secondly, refer to documentation for various label display options for a mat-form-field.
To achieve your desired outcome, consider adding the following snippet to your primary module:
@NgModule({
....
providers: [
...
{provide: MAT_FORM_FIELD_DEFAULT_OPTIONS, useValue: {floatLabel: 'always'}}
]
})
...