For mouse, BioC offers both entrezg- and ensembl-centric TxDbs:

    # Entrezg
        txdb <- TxDb.Mmusculus.UCSC.mm10.knownGene::TxDb.Mmusculus.UCSC.mm10.knownGene
        GenomicFeatures::genes(txdb)[c('100009600', '99889', '99982')]

    # Ensembl
        txdb <- TxDb.Mmusculus.UCSC.mm10.ensGene::TxDb.Mmusculus.UCSC.mm10.ensGene
        GenomicFeatures::genes(txdb)[c('ENSMUSG00000000001', 'ENSMUSG00000000003')]

For human, BioC has entrezg- but not ensembl-centric TxDbs, why is this?

   # Entrezg
        txdb <- TxDb.Hsapiens.UCSC.hg38.knownGene::TxDb.Hsapiens.UCSC.hg38.knownGene
        GenomicFeatures::genes(txdb)[c('1', '10', '100')]

   # Ensembl
       # No TxDb.Hsapiens.UCSC.hg38.ensGene::TxDb.Hsapiens.UCSC.hg38.ensGene

There isn't an ensGene table for hg38. If you hit the Track dropdown, you won't see an Ensembl Track. If you change to hg37, it's there.

And you should probably not use UCSC data for Ensembl anyway. Although both UCSC and Ensembl start with the GRCh38 genome, they use different pipelines to say what is and isn't a gene, and their ending products have many discrepancies. If you were to use the ensGene table (if it existed), you would be in essence asking for just those genes that both Ensembl and NCBI/UCSC agree are the same thing. And anything for which they disagree gets dropped.

You are better off using Johannes Rainier's EnsDb packages.

> library(AnnotationHub)

> hub <- AnnotationHub()
snapshotDate(): 2019-10-08
> query(hub, c("Homo sapiens","EnsDb"))
AnnotationHub with 13 records
# snapshotDate(): 2019-10-08 
# $dataprovider: Ensembl
# $species: Homo sapiens
# $rdataclass: EnsDb
# additional mcols(): taxonomyid, genome, description,
#   coordinate_1_based, maintainer, rdatadateadded, preparerclass, tags,
#   rdatapath, sourceurl, sourcetype 
# retrieve records with, e.g., 'object[["AH53211"]]' 

            title                            
  AH53211 | Ensembl 87 EnsDb for Homo Sapiens
  AH53715 | Ensembl 88 EnsDb for Homo Sapiens
  AH56681 | Ensembl 89 EnsDb for Homo Sapiens
  AH57757 | Ensembl 90 EnsDb for Homo Sapiens
  AH60773 | Ensembl 91 EnsDb for Homo Sapiens
  ...       ...                              
  AH67950 | Ensembl 95 EnsDb for Homo sapiens
  AH69187 | Ensembl 96 EnsDb for Homo sapiens
  AH73881 | Ensembl 97 EnsDb for Homo sapiens
  AH73986 | Ensembl 79 EnsDb for Homo sapiens
  AH75011 | Ensembl 98 EnsDb for Homo sapiens
> ensdb <- hub[["AH75011"]]
downloading 1 resources
retrieving 1 resource
  |======================================================================| 100%

loading from cache
require("ensembldb")
> ensdb
EnsDb for Ensembl:
|Backend: SQLite
|Db type: EnsDb
|Type of Gene ID: Ensembl Gene ID
|Supporting package: ensembldb
|Db created by: ensembldb package from Bioconductor
|script_version: 0.3.4
|Creation time: Mon Sep 30 20:28:27 2019
|ensembl_version: 98
|ensembl_host: localhost
|Organism: Homo sapiens
|taxonomy_id: 9606
|genome_build: GRCh38
|DBSCHEMAVERSION: 2.1
| No. of genes: 67946.
| No. of transcripts: 250194.
|Protein data available.