If it's Last 16 with sides playing sides in the same pool it's:

Leinster (1) v Sale (8)
WASPS (2) v Toulon (7)
Bordeaux (3) v Edinburgh (6)
La Rochelle (4) v Scarlets (5)
Lyon (1) v Gloucester (8)
Racing 92 (2) v Exeter (7)
Toulouse (3) v Bristol (6)
Munster (4) v Clermont (5)
QFs:

Leinster (1) or Sale (8) v Munster (4) or Clermont (5)
WASPS (2) or Toulon (7) v Toulouse (3) or Bristol (6)
Racing 92 (2) or Exeter (7) v Bordeaux (3) or Edinburgh (6)
Lyon (1) Gloucester (8) v La Rochelle (4) or Scarlets (5)

The highest ranked side would host the QF.
SFs

Leinster (1), Sale (8), Munster (4) or Clermont (5) v Racing 92 (2), Exeter (7), Bordeaux (3) or Edinburgh (6)

Lyon (1), Gloucester (8), La Rochelle (4) or Scarlets (5) v WASPS (2), Toulon (7), Toulouse (3) or Bristol (6)

Highest seed would have home country advantage.
If the last 16 with sides from opposite pools:

Leinster (1) v Gloucester (8)
WASPS (2) v Exeter (7)
Bordeaux (3) v Bristol (6)
La Rochelle (4) v Clermont (5)
Munster (4) v Scarlets (5)
Toulouse (3) v Edinburgh (6)
Racing 92 (2) v Toulon (7)
Lyon (1) v Sale (8)
QFs:

Leinster (1) or Gloucester (8) v Munster (4) or Scarlets (5)
WASPS (2) or Exeter (7) v Toulouse (3) or Edinburgh (6)
Racing 92 (2) or Toulon (7) v Bordeaux (3) or Bristol (6)
Lyon (1) or Sale (8) v La Rochelle (4) or Clermont (5)

Highest ranked side hosts.
SFs:

Leinster (1), Gloucester (8), Munster (4) or Scarlets (5) v Racing 92 (2), Toulon (7), Bordeaux (3) or Bristol (6)
Lyon (1), Sale (8), La Rochelle (4) or Clermont (5) v WASPS (2), Exeter (7), Toulouse (3) or Bristol (6).

Highest seed has home country advantage.
Re. Semi final venues, in 2019 EPCR changed the rules so that clubs with home stadiums of a sufficient capacity. The capacity isnt specified, but Racing 92 would have hosted their SF at their new stadium if they won the QF. The Ricoh can hold 2k more.
Therefore, Wasps would play at the Ricoh in the Last 16 and QF and should they make the SF and Lyon don't, they'll also be at the Ricoh for the SF too.
Challenge Cup last 16:

London Irish (1) v Glasgow (12)
Ospreys (2) v Dragons (11)
Cardiff (3) v Harlequins (10)
Leicester (4) v Montpellier (9)
Bath (8) v Zebre (13)
Connacht (7) v Agen (14)
Northampton (6) v Benetton (15)
Ulster (5) v Newcastle (16)

Rank in brackets.
QF:

London Irish (1) or Glasgow (12) v Bath (8) or Zebre (13)
Cardiff (3) or Harlequins (10) v Northampton (6) or Benetton (15)
Ospreys (2) or Dragons (11) v Connacht (7) or Agen (14)
Ulster (5) or Newcastle (16) v Leicester (4) or Montpellier (9)

Highest rank hosts the QF
SF:

London Irish (1), Glasgow (12), Bath (8) or Zebre (13) v Cardiff (3), Harlequins (10), Northampton (6) or Benetton (15)

Ospreys (2), Dragons (11), Connacht (7) or Agen (14) v Ulster (5), Newcastle (16), Leicester (4) or Montpellier (9)

Highest ranked side hosts the SF.
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